CBSE Class 8

These NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Chemical Effects of Electric Current NCERT Solutions for Class 8 Science Chapter 14

Class 8 Science Chapter 14 Chemical Effects of Electric Current Textbook Exercise Questions and Answers

Page 180-181

Question 1.
Fill in the blanks:
a. Most liquids that conduct electricity are solutions of ……………., ……………. and …………….
b. The passage of an electric current through a solution causes ……………. effects.
c. If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ……………. terminal of the battery.
d. The process of depositing a layer of any desired metal on another material by means of electricity is called …………….
Answer:
a. acids, bases and salts
b. chemical
c. negative
d. electroplating

Question 2.
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
Answer:
The deflection in the compass needle shows that current is flowing through the wound up wire and hence, through the circuit. This shows that the solution is a conducting solution because only then the current can flow through it through the circuit. This current through the wound up wire produces a magnetic field around it which acts on the magnetic needle of the compass and deflects it.

Question 3.
Name three liquids, which when tested in the manner shown in the figure, may cause the magnetic needle to deflect.

Answer:
Liquids like lemon juice, saltwater and vinegar can conduct electricity. Hence, these liquids can cause the magnetic needle to deflect.

Question 4.
The bulb does not glow in the setup shown in the figure. List the possible reasons. Explain your answer.

Answer:
The bulb may not glow because of the following reasons:

• Liquid in the beaker may be non-conducting. In such a case, the electric current would not be able to pass through the liquid. Hence, the circuit will not be complete.
• The conductivity of the liquid may be very low and so the current flowing through the circuit may be too weak to produce enough heat in the filament of the bulb to make it glow.
• The battery may be exhausted and may not have sufficient energy to generate electricity.
• The bulb may be fused or the circuit connections may be loose.

Question 5.
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
a. liquid A is a better conductor than liquid B
b. liquid B is a better conductor than liquid A
c. both liquids are equally conducting
d. conducting properties of liquid cannot be compared in this manner
Answer:
a. liquid A is a better conductor than liquid B.

Question 6.
Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer:
No, pure water does not conduct electricity. This is because pure water is devoid of any salts. Pure water can conduct electricity when a pinch of salt is added to it, as salt solution is conducting in nature.

Question 7.
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.
Answer:
Normal water is a good conductor of electricity. Therefore, firemen shut off the main electrical supply for the area before using the water hoses in the case of fire to prevent themselves from getting an electric shock.

Question 8.
A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?
Answer:
Water we use for drinking purposes has very less amount of salts dissolved in it. However, seawater has very high concentration of salts in it and hence is a greater conductor of electricity than drinking water. Therefore, more current flows through seawater and the compass needle shows more deflection in this case.

Question 9.
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Answer:
Rainwater may contain many impurities which get dissolved in it when it passes through the various layers of troposphere. Due to this, rainwater becomes a good conductor of electricity. If an electrician carries out electrical repairs outdoors during heavy downpour, he may get an electric shock. Hence, it is not safe for the electrician to carry out electrical repairs outdoors during heavy downpour.

Question 10.
Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons?
Answer:
Rainwater is a pure water but many impurities and acidic gases get dissolved in the rainwater while it falls to the earth through the atmosphere. Due to the presence of these impurities and small amounts of acids in it, the rainwater conducts electricity. This explains Paheli’s observation that rainwater can allow electricity to pass through it while distilled water cannot.

Question 11.
Prepare a list of objects around you that are electroplated.
Answer:
Examples of electroplated objects are as follows:

• Chromium plating is done on different parts of cars, buses and motorcycles to give them shiny appearance.
• A fine layer of gold is deposited on the silver ornaments, and they are called gold-plated ornaments.
• Iron used in constructing a building is coated with a layer of zinc (galvanisation). This protects iron from corrosion and rusting.

Question 12.
The process that you saw in Activity 14.7 (of NCERT textbook) is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?
Answer:
The thin plate of pure copper should be connected to the positive terminal of the battery. This is because when electric current is passed through the copper sulphate solution, it dissociates into copper and sulphate. The free copper is drawn towards the negative terminal of the battery and gets deposited on it. On the other hand, the loss of copper from the solution would be regained from the impure copper rod which is connected to the positive terminal of the battery.

NCERT Extended Learning Activities and Projects

Question 1.
Test the conduction of electricity through various fruits and vegetables. Display your result in a tabular form.
Hint:
Fruits such as oranges, apples, peach and grapes are good conductors of electricity whereas fruits like kiwi, banana, papaya, pineapple are poor conductors of electricity. On the other hand, vegetables such as lemon, tomatoes, carrot and reddish are good conductors of electricity whereas onion, cabbage, cauliflower are poor conductors of electricity.

Question 2.
Repeat Activity 14.7 (of NCERT textbook) with a zinc plate in place of the copper plate connected to the negative terminal of the battery. Now replace zinc plate with some other metallic object and again repeat the activity. Which metal gets deposited over which other metal? Discuss your findings with your friends.
Hint:
In each case copper gets deposited over the metal plate because the electrolyte used is copper sulphate and the positive electrode is of copper.

Question 3.
Find out if there is a commercial electroplating unit in your town. What objects are electroplated there and for what purpose? (The process of electroplating in a commercial unit is much more complex than what we did in Activity 14.7). Find out how they dispose off the chemicals they discard.
Hint:
Do it yourself.

Question 4.
Imagine that you are an ‘entrepreneur’ and have been provided a loan by a bank to set up a small electroplating unit. What object would you like to electroplate and for what purpose? (Look up the meaning of ‘entrepreneur’ in a dictionary).
Hint:
If I were an entrepreneur and was provided a loan by a bank to set up a small electroplating unit, I would start an electroplating unit of electronic and electrical appliances because:

• There is an increasing demand for electronic devices.
• The telecommunication industry is ever-expanding.
• Cost efficiency of electroplating these articles is high as compared to other technologies.

Question 5.
Find out the health concerns associated with chromium electroplating. How are people trying to resolve them?
Hint:
The health problems associated with chromium electroplating are skin rashes, stomach ulcers, respiratory problems, weak immunity, kidney and liver damage and cancer. Steps taken to resolve it may include use of water insoluble chromium compounds and mild steel.

Question 6.
You can make a fun pen for yourself. Take a conducting metal plate and spread a moist paste of potassium iodide and starch. Connect the plate to a battery as shown in the figure. Now using the free end of the wire, write a few letters on the paste. What do you see?

Hint:
Do it yourself.

Activity 1

Objective: To test the electrical conductivity of lemon juice (acid) or vinegar.
Materials Required: A few small plastic or rubber caps of discarded bottles, lemon juice/ vinegar, tester.
Procedure:

• Collect a few small plastic or rubber caps of discarded bottles and clean them.
• Pour one teaspoonful of lemon juice or vinegar in one cap.
• Bring your tester over this cap and let the ends of the tester dip into the lemon juice or vinegar. Take care that the ends are not more than 1 cm apart and at the same time do not touch each other. See what happens.

Observation: On letting the ends of tester dip into the lemon juice or vinegar, the bulb starts glowing.

Conclusion: This activity proves that both of the lemon juice and the vinegar are good conductors of electricity.

Liquids hich are Good Conductors: Some liquids which can conduct electricity are tap water lemon juice, vinegar, salt solution. Most of the liquids which conduct electricity are solutions of acids, bases or salts.

Liquids which are Poor Conductors: Some liquids are bad conductors of electricity, e.g.. distilled water, honey, milk. vegetable oil, etc.

Water: Tap water conducts electricity because it contains various salts dissolved in it. Hence, it is advised not to touch an electric switch with wet hands. Salt water also conducts electricity. However, distilled water is pure and free of salts, hence is a poor conductor.

Activity 2

Objective: To show that the distilled water is a poor conductor of electricity.
Materials Required: Distilled water, plastic or rubber cap.

Procedure:

• Take about two teaspoonful of distilled water in a clean and dry plastic or rubber cap of a bottle.
• Use the tester to test whether distilled water conducts electricity or not.
• Now dissolve a pinch of common salt in distilled water and test again. Record your observations.

Observation: The bulb of the tester does not glow when tester is put in distilled water. This means that distilled water does not conduct electricity. When tester is put in water dissolved with a pinch of salt, the bulb of the tester glows. This shows that when a pinch of salt is dissolved in distilled water, it conducts electricity.
Conclusion: Distilled water is a poor conductor of electricity.

Heating Effect of Electric Current:
The heating effect of electric current is responsible for the glowing of the bulb. When current passes through the bulb, the filament gets heated to a high temperature and as a result the bulb starts glowing. But, if current is very small, the filament will not get heated to a high temperature and will not glow.

Magnetic Effect of Electric Current:
When electric current passes through a conductor, a magnetic field is created around it. This is called magnetic effect of electric current. This can be demonstrated by keeping a magnetic compass near a current-carrying conductor. A magnetic compass shows deflection when it is placed near a current-carrying conductor. The deflection is the indicator of the presence of magnetic field.

Chemical Effects of Electric Current: A British Chemist named William Nicholson performed an experiment showing that if current is passed through water, bubbles of oxygen and hydrogen are produced. The oxygen bubbles will be present on the positive electrode and hydrogen bubbles on the negative electrode. The passage of an electric current through a conducting solution causes chemical reactions, due to which bubbles of a gas may be formed on the electrodes. Hence, it can be said that when electric current is passed through a conducting solution, some chemical reaction takes place in the solution. This is called the chemical effect of electric current. Some of the chemical effects of electric current are as follows:

• Bubbles of a gas may be formed on the electrodes.
• Deposits of metal may be seen on electrodes.
• Change of colour of solution may occur.

Potato tester: Current produces chemical effects in fruits and vegetables also. When the two ends of a tester are inserted inside a cut potato, the end connected to the positive terminal of the battery develops greenish-blue spot on the potato. This can be used to identify the positive terminal of a cell or battery concealed in a box.

Electroplating: The process of depositing a layer of a desired metal on any other material by means of electricity is called electroplating. Following steps are performed in electroplating:

• The material which needs to be coated with the desired layer is made the negative electrode (cathode).
• The plate of desired metal is made the positive electrode (anode).
• The conducting solution is made of a salt of desired metal.
• Electric current is passed through the solution.
• The desired metal dissociates from the plate (anode) and gets deposited on the material at negative electrode (cathode).

Electroplating of copper: During electroplating of copper, the copper plates are made the electrodes and copper sulphate solution is used. When electric current is passed through copper sulphate solution, copper sulphate dissociates into copper and sulphate. The free copper gets drawn to the negative electrode and gets deposited there. From the other electrode, an equal amount of copper gets dissolved in the solution and the process continues.

Activity 3

Objective: To show the process of electroplating.
Materials Required: Two copper plates of size around 10 cm x 4 cm, copper sulphate, distilled water, beaker, dilute sulphuric acid, sandpaper and battery.
Procedure:

• Take 250 mL of distilled water in a clean and dry vessel.
• Dissolve two teaspoonful of copper sulphate in it.
• Add a few drops of dilute sulphuric acid to the copper sulphate solution to make it more conducting.
• Clean the copper plates with sand paper.
• Now rinse them with water and dry them.
• Connect the copper plates to the terminals of battery and immerse them in copper sulphate solution.
• Allow the current to pass through the circuit for about 15 minutes.
• Now remove the electrodes from the solution and observe them carefully.

Observation: Copper metal gets deposited on the plate at negative terminal of the battery.

Conclusion: When the current is passed through the copper sulphate solution, copper gets deposited on the plate acting as cathode.

Applications of Electroplating:
i. Imitation jewellery is made by applying a layer of gold or silver.
ii. Applying a layer of chromium on an article by this method is called chrome plating. Parts of bicycle, motorbike and sanitary fittings are chrome plated by this method. Chromium is shiny, and resists corrosion and scratches.
iii. Tin cans are made by electroplating tin on iron.
iv. Applying a natural oxide layer on an article is called anodising. Electric poles and beams on bridges are electroplated with zinc. Chrome plating and anodising help in preventing articles from corrosion.

v. Galvanisation is a process in which a coating of zinc is deposited on iron to protect it from corrosion and formation of rust.

Problem of Electroplating: The disposal of the conducting solution of electroplating factories is a major problem. It is a polluting waste, and there are specific disposal guidelines which should be followed to protect the environment.

Class 8 Science Chapter 14 Chemical Effects of Electric Current Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Define good conductors of electricity.
Answer:
Electric current can easily pass through some materials. Such materials are called good conductors of electricity, e.g., iron, copper, silver, aluminium, gold, etc.

Question 2.
Define bad conductors of electricity.
Answer:
Electric current cannot pass easily through some materials. Such materials are called bad conductors of electricity, e.g., rubber, wood, asbestos, plastic, etc.

Question 3.
Why is zinc electroplated on iron?
Answer:
Zinc is electroplated on iron to prevent it from rusting.

Question 4.
Why is distilled water a poor conductor of electricity?
Answer:
Because distilled water is free from any kind of salts.

Question 5.
What effect does the current produce when it flows through a conducting solution?
Answer:
Current causes chemical reaction when it flows through a conducting solution.

Question 6.
What is galvanisation?
Answer:
Applying a layer of zinc on an iron article by electroplating is called galvanisation.

Question 7.
Why is it dangerous to touch an electrical appliance with wet hands?
Answer:
This is because water can conduct electricity.

Question 8.
What is LED?
Answer:
It is a semiconductor light source, used to detect a weak current in the circuit.

Question 9.
During electrolysis of water, oxygen gas is deposited at which electrode?
Answer:
Positive terminal or anode.

Question 10.
During electrolysis of water, hydrogen gas is deposited at which electrode?
Answer:
Negative terminal or cathode.

Question 11.
What is an electrolyte?
Answer:
A substance which conducts electricity in the liquid state or when dissolved in water and breaks up chemically during the process is called an electrolyte.

Question 12.
What is the practical use of LED?
Answer:
These are used in traffic lights and increasingly being used for lightning.

Question 13.
Define electric current.
Answer:
The amount of charge flowing per unit time is called electric current.

Question 14.
Name a liquid which does not conduct electricity.
Answer:
Distilled water.

Question 15.
What is the full form of LED?
Answer:
Light emitting diode.

Question 16.
Which of these do you think uses less electrical energy-electric bulb or LED?
Answer:
LED uses less electrical energy.

Short Answer Type Questions

Question 1.
Explain the heating effect of electric current.
Answer:
When an electric current passes through a conductor, it increases the temperature of the conductor. This is called heating effect of electric current. Many electrical appliances work on heating effect of electric current, e.g., electric bulb, water heater, electric iron, etc.

Question 2.
Explain the magnetic effect of electric current.
Answer:
When an electric current passes through a conductor, a magnetic field is created around the conductor. This is called magnetic effect of electric current. This can be demonstrated by keeping a magnetic compass near a current-carrying conductor. A magnetic compass shows deflection when it is placed near a current-carrying conductor. The devices which work on magnetic effect of electric current are electric motor and electromagnet.

Question 3.
What happens when electrodes are immersed in water and current is passed through it?
Answer:
When electrodes are immersed in water and current is passed through it, bubbles of oxygen and hydrogen gases are produced. Oxygen bubbles are formed at the electrode connected to the positive terminal. Hydrogen bubbles are formed at the electrode connected to the negative terminal.

Question 4.
Differentiate between conductors and insulators.
Answer:

Question 5.
Describe an electrical tester.
Answer:
An electrical tester is a simple piece of electronic test equipment used to determine the presence or absence of an electric voltage in a piece of equipment under test. It is also used to test whether a liquid allows electric current to pass through it or not.

Question 6.
Why is water that we get from ponds and hand pumps always a good conductor of electricity?
Answer:
Water that we get from ponds and hand pumps is not pure. It contains several amounts of mineral salts naturally dissolved in it, and thus is a good conductor.

Question 7.
Discuss what happens when an electrolyte is dissolved in water, and then an electric current is passed through it.
Answer:
When an electrolyte is dissolved in water, it breaks up into cations (positively charged ions) and anions (negatively charged ions). When an electric current is passed through the solution, the cations move towards the cathode (negatively charged electrode) and the anions move towards anode (positively charged electrode). This results in a chemical reaction or a chemical change.

Question 8.
What is the application of the chemical effects of electricity in our daily life? Give examples.
Answer:
Examples of chemical effect are as follows:

1. Electroplating: A layer of one metal is coated on the other substance or metal by the effect of electric current. This is called electroplating.
2. Electrolysis: The process by which a compound is decomposed into its constituents under the effect of electric current is called electrolysis.

Question 9.
List some of the chemical effects of electric current.
Answer:
When electric current is passed through a conducting solution, some chemical reaction takes place in the solution. This is called chemical effect of electric current. Some of the chemical effects of current are as follows:

• Bubbles of a gas may be formed at the electrodes.
• Deposits of metal may be seen on electrodes.
• Change of colour of solution may occur.

Question 10.
It is quite possible that even though a liquid allows electric current to pass through it and the circuit is complete but still the bulb does not glow? Why?
Answer:
When current passes through a substance, it offers opposition to the flow of current. If the opposition offered by the substance is small, the current will be large and if the opposition is large, the current will be small. However, if the current through the circuit is too small, the filament of the bulb does not get heated up to a sufficient temperature and it does not glow.

Long Answer Type Questions

Question 1.
Write the steps involved in electroplating.
Answer:
Following steps are involved in electroplating:

• The material which needs to be coated with a layer of desired metal is made the negative electrode (cathode).
• The plate of desired metal is made the positive electrode (anode).
• The conducting solution is made of a salt of desired metal.
• Electric current is passed through the solution.
• The electrolyte splits up releasing the ions of desired metal which get deposited at the cathode.
• An equal amount of the metal dissociates from the plate (anode) and gets dissolved in the electrolyte.

Question 2.
Write some uses of electroplating.
Answer:
Some uses of electroplating are as follows:

• Imitation jewellery is made by applying a layer of gold or silver.
• Applying a layer of chromium on an article by this method is called chrome plating. Parts of bicycle, motorbike and sanitary fittings are chrome plated by this method.
• Tin cans are made by electroplating tin on iron.
• Applying a layer of zinc on an article is called galvanisation.

Electric poles and beams on bridges are electroplated with zinc.

Question 3.
How will you show experimentally that an electric current can bring about a chemical change? What is this phenomenon called?
Answer:
Take two iron nails. Clean them with sandpaper. Wrap one or two rounds of copper wire around them and connect the other ends of the wires to the two terminals of an electric battery. Take water in a beaker and add to it a little salt or a few drops of sulphuric acid to make it conducting. Immerse the nails (called electrodes) in the solution. Observe the nails carefully.

You can see small bubbles of gases coming out from the water near the nails. It can be checked that the gases evolved are hydrogen and oxygen. The gases come from water-electric current breaks up water into its constituent gases, hydrogen and oxygen. This observation, therefore, shows that electric current has a chemical effect on water. This experiment shows that an electric current can bring about a chemical change. This phenomenon is called electrolysis.

Question 4.
Describe how you will prepare a tester to test conduction based on the magnetic effect of electricity.
Answer:
Take a tray of a discarded matchbox. Wrap an electric wire a few times around the tray. Place a smalt compass needle inside it. Now connect one free end of the wire to the terminal of a battery. Leave the other end free. Take another piece of wire and connect it to the other terminal of the battery. Join the free ends of the two wires momentarily. The compass needle should show deflection. The tester with two free ends of the wire is ready.

Question 5.
Complete the following table.

Answer:

Picture-Based Questions

Question 1.
Draw a well labelled diagram of passing current through water and answer the following questions.
a. Which material is used as electrodes?
b. Name the material used to make caps of electrodes.
c. Name the gases formed.
d. Can we call the change in water as a chemical change?
Answer:

a. Carbon rod is used as electrodes.
b. Brass caps or any other metal.
c. Oxygen and hydrogen.
d. Yes, it is a chemical change.

Question 2.
Observe the following circuits carefully. In which circuit will the bulb glow?

Answer:
a. No, because a rubber band is a poor conductor of electricity.
b. Yes, because iron key is a good conductor of electricity.
c. No, because pen is made of plastic which is a poor conductor of electricity.
d. Yes, because safety pin made of steel is a good conductor of electricity.

These NCERT Solutions for Class 8 Science Chapter 13 Sound Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Sound NCERT Solutions for Class 8 Science Chapter 13

Class 8 Science Chapter 13 Sound Textbook Exercise Questions and Answers

Page 168-169

Question 1.
Sound can travel through
a. gases only
b. solids only
c. liquids only
d. solids, liquids and gases
Answer:
d. solids, liquids and gases

Question 2.
Voice of which of the following is likely to have minimum frequency?
a. Baby girl
b. Baby boy
c. A man
d. A woman
Answer:
c. A man

Question 3.
In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false:
a. Sound cannot travel in vacuum. (T/F)
b. The number of oscillations per second of a vibrating object is called its time period. (T/F)
c. If the amplitude of vibration is large, sound is feeble. (T/F)
d. For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
e. The lower the frequency of vibration, the higher is the pitch. (T/F)
f. Unwanted or unpleasant sound is termed as music. (T/F)
g. Noise pollution may cause partial hearing impairment. (T/F)
Answer:
a. True
b. False
c. False
d. True
e. False
f. False
g. True

Question 4.
Fill in the blanks with suitable words,
a. Time taken by an object to complete one oscillation is called ………………
b. Loudness is determined by the ……………… of vibration.
c. The unit of frequency is ………………
d. Unwanted sound is called ………………
e. Shrillness of a sound is determined by the ……………… of vibration.
Answer:
a. time period,
b. amplitude,
c. hertz,
d. noise,
e. frequency

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
Number of oscillations = 40
Total time taken = 4 seconds
Time period = Time taken for one oscillation
= \(\frac{\text { Total time }}{\text { Total number of oscillations }}\)
= \(\frac{4 \text { seconds }}{40}\) = \(\frac {1}{10}\)second = 0.1 second
Frequency of oscillations is defined as the number of oscillations of a vibrating body per second.
Frequency = \(\frac{\text { Number of oscillations }}{\text { Total time }}\) = \(\frac {40}{4}\) = 10 Hz.

Question 6.
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer:
The time required to complete on oscillation is known as time period. It is given by the inverse of the frequency.
Time period = \(\frac{1}{\text { Frequency of oscillations }}\)
Frequency of oscillations = 500 Hz
Time period = \(\frac {1}{500}\) = 0.002 s

Question 7.
Identify the part which vibrates to produce sound in the following instruments:
a. Dholak
b. Sitar
c. Flute
Answer:
a. Stretched membrane,
b. String,
c. Air column

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Answer:
Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music. Music can become noise at many instances. When someone tries to enjoy very loud music, it can become noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Question 9.
List sources of noise pollution in your surroundings.
Answer:
Some sources of noise pollution are as follows:

• Television and transistors running at high volumes
• Loudspeakers and crackers
• Homs of buses, cars and trucks
• Home appliances such as mixer, desert cooler, etc.

Question 10.
Explain in what way noise pollution is harmful to human.
Answer:
Noise pollution is harmful to humans in many ways. Constant exposure to noise pollution can create many health related problems like insomnia, hypertension and may even lead to loss of hearing.

Question 11.
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer:
I will suggest my parents to buy a house which is away from the roadside, because house at the roadside would be noisy due to running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter and healthier to live in.

Question 12.
Sketch larynx and explain its function in your own words.
Answer:
Larynx is a part of throat responsible for the production of sound. A sketch of a human larynx is shown as follows:

Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.

Question 13.
Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Answer:
We know that the speed of light is much more than the speed of sound. Due to this, light reaches us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.

NCERT Extended Learning Activities and Projects

Question 1.
Visit the music room of your school. You may also visit musicians in your locality. Make a list of musical instruments. Note down the parts of these instruments that vibrate to produce sound.
Hint.
There are membrane instruments like tabla, mridangam and drums which produce sound when the membrane is struck. There are wind instruments like flute, trumpet and saxophone which produce sound by the vibrating air columns. There are string instruments like violin, veena and guitar which produce sound when the string is struck.

Question 2.
If you play a musical instrument, bring it to the class and demonstrate how you play it.
Hint.
Do it yourself.

Question 3.
Prepare a list of famous Indian musicians and the instruments they play.
Hint:

Question 4.
Take a long thread. Place your hands over your ears and get someone to place this thread round your head and hands. Ask her to make the thread taut and hold its ends in one hand. Now ask her to draw her finger and thumb tightly along the thread. Can you hear a rolling sound like that of a thunder? Now repeat the activity while another friend stands near both of you. Can he hear any sound?

Hint.
Do it yourself.

Question 5.
Make two toy telephones. Use them as shown in the given figure. Make sure that the two strings are taut and touch each other. Let one of you speak. Can the remaining three persons hear? See how many more friends you can engage in this way. Explain your observations.

Hint.
Do it yourself.

Question 6.
Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbours. Suggest how to control noise pollution. Prepare a brief report and present it in the class.
Hint.
The various sources of noise pollution are television, transistor, loudspeakers, crackers, horns of vehicles, home appliances, etc. Noise pollution can be controlled by planting tree belts, using silencers in automobile engines, sound barriers, etc.

Activity 1

Objective: To show that a vibrating body produces sound.
Materials Required: A metal plate or a frying pan and a stick.
Procedure:

• Take a frying pan.
• Hang it at a convenient place in such a way that it does not touch any wall.
• Now strike it with a stick.
• Touch the frying pan gently with your finger, and try to feel the vibrations.
• Touch the frying pan after it stops producing sound. Try to feel the vibrations again.

Observation: On striking the frying pan, it produces sound and we feel vibrations on touching it. But when it stops producing sound, there are no vibrations at all.
Conclusion: Sound is produced by a vibrating object. If an object does not vibrate, it cannot produce sound.

Sound Produced by Humans: Sound is produced by the voice box or larynx in human beings. The voice box or larynx is situated in the upper part of windpipe. There are two vocal cords stretched across a small gap between them. When air is forced through the cords, they begin to vibrate and sound is produced. Muscles which are attached to the vo’cal cords enable us to make the vocal cords tight or loose as per need. Sound quality varies according to the tension or slack in the vocal cords. The vocal cords in males are of length 20 mm, and females have 15 mm long vocal cords. Children, on the other hand, have very short lengthed vocal cords. Hence, the voices, their quality and their type are always different in women, men and children.

Propagation of sound: The travelling of sound from source of receiver is called propagation of sound. Sound is propagated by the to and fro motion of particles of the medium.

Sound needs a medium to propagate: A medium is necessary for the propagation of sound waves. The matter or substance through which sound is transmitted is called a medium. The medium can be solid, liquid or gas. Sound cannot travel in vacuum. A true vacuum refers to the complete absence of matter.

Activity 2

Objective: To show that sound travels through liquids.
Materials Required: A bucket or a bathtub, clean water, a small bell.
Procedure:

• Take a bucket or a bathtub.
• Fill it with clean water.
• Take a small bell in one hand.
• Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub.
• Place your ear gently on the water surface.
• Try to hear the sound of the bell.

Observation: On shaking the bell in water and placing the ear on water surface, the sound of bell is heard.

Conclusion: Sound travels through liquids.

We hear sound through our ears: Sounds are produced as waves in the air or any other medium. These” waves are converted into electrical signals or messages that our brain can understand. Our ears have a special structure that allows this function. There are three major parts of the human ear:

i. Outer ear: It consists of ear pinna and a canal at the end of which is a stretched membrane, called eardrum. Pinna collects the sound waves which then pass through the ear canal and causes vibrations in eardrum.
ii. Middle ear: It has a set of three bones which are linked together and attached to the eardrum. These bones receive vibrations from the eardrum, amplify them and pass on to the inner ear.
iii. Inner ear: The inner ear receives vibrations from the middle ear and changes them into electrical signals. These signals are carried by the auditory nerves to the brain. The brain interprets these signals as sound.

Amplitude, Time Period and Frequency of a Vibration:
A repeated ‘back and forth’ motion is called vibration (or oscillation). When an object moves back and forth continuously, we say that it is under oscillatory motion. For example, when a swing moves back and forth repeatedly, we say that the swing is making oscillations.

i. Amplitude: The amplitude of a sound wave can be defined as the maximum displacement of the particles from their mean position due to the vibration.

ii. Time period: The time taken for one complete oscillation of a sound wave is called the time period of the sound wave.
iii. Frequency of oscillation: The number of oscillations an object completes per second is called its frequency. The SI unit of frequency is Hertz (Hz).
1 Hz = 1 oscillation per second
20 Hz = 20 oscillations per second

Loudness of Sound: Loudness of sound depends on the amplitude of vibration. Loudness of sound is directly proportional to the square of amplitude of vibration. Louder sound has higher amplitude while quieter sound has lower amplitude. Loudness is expressed in terms of decibel.

The following table gives the loudness of sound from various sources:

Activity 3

Objective: To show that the loudness of sound depends upon the amplitude of vibration.
Materials Required: A metallic tumbler, a table spoon, a small thermocol ball.
Procedure:

• Take a metallic tumbler and a table spoon.
• Now suspend a small thermocol ball touching the rim of the tumbler.
• Vibrate the tumbler by striking it with spoon.
• See how far the ball is displaced. The displacement of the ball is a measure of the amplitude of vibration of the tumbler.
• Now, strike the tumbler (a) gently and then (b) with some force.
• Compare the amplitudes of vibrations of the tumbler in the two cases.

Observation: The amplitude of sound is larger in the first case (a) as compared to the second case (b) as the ball was displaced more in case (a).
Conclusion: The loudness of sound depends upon the amplitude of vibration.

Pitch or Shrillness: Pitch of sound depends on the frequency of vibration. A high pitched sound has high frequency, while a low pitched sound has low frequency. Children and women generally produce sound with a high pitch.

Audible and Inaudible Sounds:
Human beings can hear sounds between frequencies 20 Hz and 20,000 Hz. This range of frequencies is called the audible range for humans. The sound with frequency below 20 Hz is called infrasound. On the other hand, the sound with frequency above 20,000 Hz is called ultrasound.

Noise and Music: A sound which is pleasant to ears is called music. However, any unpleasant sound is called noise.

Noise Pollution: Presence of excess noise in the environment is called noise pollution. Automobiles, factories, loud music, construction works, firecrackers, stone quarry, etc., are all sources of noise pollution.

Effects of Noise Pollution: Continuous exposure to noise pollution can result in lack of sleep (insomnia), hypertension (high blood pressure), anxiety and many other disorders. Noise pollution can also lead to partial loss of hearing in some cases.

Measures to Limit Noise Pollution:

• Aircraft engines and automobile engines should be fitted with silencing devices. The muffler (or silencer) in a motorcycle is an example of such device.
• Factories should be relocated far away from the residential areas. Many factories from Delhi had been shifted to outskirts in the 1990s.
• Trees should be planted along the roads because trees absorb sound waves or noise.
• Sound barriers should be installed along flyovers.

Hearing Impairment: Loss of hearing is called hearing impairment. It can be total or partial, but total hearing impairment is rare and is usually congenital, i.e., by birth. A person with hearing impairment can learn sign language to communicate with others. Hearing aids can be used by people who are suffering from partial hearing impairment.

Class 8 Science Chapter 13 Sound Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is sound?
Answer:
Sound is a form of energy which gives the sensation of hearing.

Question 2.
How does sound travel from one place to another?
Answer:
Sound travels through a medium, like solid, liquid or gas by the vibrations of the particles of medium. It cannot travel through vacuum.

Question 3.
Define frequency.
Answer:
The number of oscillations per second is called frequency of oscillation.

Question 4.
What is the unit of frequency?
Answer:
Hertz.

Question 5.
What are the two important properties of sound?
Answer:
Amplitude and frequency.

Question 6.
Name the factor on which the loudness of the sound depends.
Answer:
Amplitude.

Question 7.
How can we minimise the noise made by transport vehicles?
Answer:
By installing silencing device in transport vehicles. Also, the use of automobile horns should be minimised for this purpose.

Question 8.
Name some health disorders caused by noise pollution.
Answer:
Hypertension, lack of sleep, anxiety, etc.

Question 9.
Name some musical instruments.
Answer:
Tabla, sitar, harmonium, flute, etc.

Question 10.
What is the frequency limit upto which bats can hear?
Answer:
Bats can hear upto a frequency of 100 kHz.

Question 11.
Define loudness. Name its unit.
Answer:
Loudness is the degree of sensation of sound produced by an object and is usually a perception of sound pressure. It is measured in a unit called decibel (dB).

Question 12.
Name the three parts of the human ear.
Answer:
Outer ear, middle ear and inner ear.

Question 13.
Name some sources of sound.
Answer:
Human vocal cords, tuning fork, stringed instruments, drums, bells, etc.

Question 14.
What do you understand by the term quality of sound?
Answer:
Factors which distinguish between various sounds are called the quality or timber of sound.

Question 15.
Define hertz.
Answer:
A frequency of one hertz is defined as one oscillation per second.

Question 16.
Define vacuum.
Answer:
Vacuum is the space that is completely devoid of matter.

Question 17.
What are the sound waves having frequency greater than 20,000 Hz called?
Answer:
Ultrasonic waves.

Question 18.
Define noise.
Answer:
Unpleasant or unwanted sounds are called noise.

Question 19.
Name the voice producing organ of the humans.
Answer:
Voice box.

Question 20.
Define oscillatory motion.
Answer:
The to and fro motion of an object is known as oscillatory motion.

Question 21.
What do you mean by hearing impairment?
Answer:
Hearing impairment refers to the hearing loss that prevents a person from receiving and processing sounds properly through the ear. If the loss is mild, the person has difficulty in hearing.

Short Answer Type Questions

Question 1.
How is sound produced?
Answer:
Sound is produced by the vibrations of a body. Vibration is the to and fro motion of a particle. These vibrations are passed on from one particle of the medium to another in the form of sound wave.

Question 2.
Which property of vibration determines the pitch of the sound? Explain giving some examples.
Answer:
Frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher, the sound has a higher pitch. If the frequency of vibration is lower, the sound has a lower pitch. Sound of women is shriller than that of men due to high frequency of sound in women.

Question 3.
What is noise pollution? What are its major causes?
Answer:
Presence of excessive or unwanted sounds due to irregular frequencies in the atmosphere is called noise pollution. Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers, etc.

Question 4.
Why is there a difference in the sound of a baby and an adult?
Answer:
The loudness of sound depends upon the amplitude of vibration. When the amplitude of vibration is large, sound produced is loud and when the amplitude of vibration is low, sound produced is feeble. A baby has comparatively shorter vocal cords and hence it produces a feeble sound with low amplitude whereas longer vocal cords of adults produce a louder voice.

Question 5.
Define the amplitude of a vibration with the help of a diagram.
Answer:
The maximum distance moved by a particle from the mean or central position in a vibrational motion is called the amplitude of that particle.

Question 6.
What are the two main properties of sound?
Answer:
The two main properties of sound are as follows:

1. Amplitude: The maximum displacement of an oscillatory body from the mean position is called amplitude. It controls the loudness of a sound; more the amplitude, more loud the sound is.
2. Frequency: The number of oscillations in one second is called frequency. It controls the pitch of the sounds. High pitched sound means higher frequency.

Question 7.
What are the characteristics of sound?
Answer:
Following are some of the characteristics of sound:

• Sound travels in longitudinal waves.
• The amplitude of sound determines its loudness.
• Tone is a measure of the quality of a sound wave.
• Sound travels faster in hot medium, and in solids.

Question 8.
Why do astronomers fail to hear the sound of each other on the surface of the moon?
Answer:
Astronomers fail to hear sound of each other while they are on the surface of the moon because there is no air or atmosphere present on the moon and we know that sound cannot travel through vacuum. It needs a medium to travel. Hence, sound cannot propagate on the surface of the moon.

Question 9.
Who has a higher pitched sound, a man or a woman? What is the reason for the difference between the two?
Answer:
Men normally have longer vocal cords than those of women. Thus, men have lower pitched voice while women have higher pitched voice.

Question 10.
What is SONAR? What is the basic principle of its working?
Answer:
SONAR refers to Sound Navigation and Ranging. The principle of reflection of sound is used in SONAR. SONAR is used to measure the depth of the ocean. Ultrasonic waves are sent from the ship down into the sea. They are received back after reflection from the sea bed. The depth is calculated by noting the time period.

Question 11.
Distinguish between high pitch sound and low pitch sound.
Answer:

Question 12.
State the conditions necessary for sound to be heard.
Answer:

1. There must be a vibrating body which should be capable of transferring its energy to its surroundings.
2. There must be a material medium to pick up the energy and then propagate it in forward direction.
3. There must be a receiver, so as to receive the sound vibration and then transmit it to the brain for final interpretation.

Question 13.
It is not advisable to construct houses near airports. In spite of that many new residential apartments are constructed near airports. Raman files RTI and also complains the municipal office about the same.
a. Why should one not reside near an airport?
b. Name other two places where there is noise pollution.
Answer:
a. Airplanes cause noise pollution while their landing and taking off. Due to this, many health problems occur like deafness, high blood pressure, etc.
b. Near the railway station and near the heavy traffic area.

Long Answer Type Questions

Question 1.
List some methods of controlling noise pollution.
Answer:
Methods of controlling noise pollution are as follows:

• Planning land use to reduce noise; for example, making tree-lined buffer zones between residential colonies and roads with heavy traffic.
• Reducing noise emissions by developing low-noise products, for example, better silencers for automobiles.
• Screens and enclosures can be made around machinery to obstruct the path of noise. This will help the people working in and living near the factories.
• Using double glazed glass windows at home to keep out noise.
• Control over recreational noise, such as use of loudspeakers.
• Increasing public awareness by providing factual information of the harmful effects of noise from blowing horns, loud music and TV, so that people themselves reduce noise generation.

However, all these measures can succeed only by the participation of public.

Question 2.
Explain the working of human ear with the help of a labelled diagram.
Answer:
The ear can be divided into outer ear, middle ear and inner ear. The outer ear consists of pinna which collects sound waves and the ear canal which conducts these waves to the eardrum. The eardrum, or tympanic membrane, is a thin, cone-shaped membrane that separates the outer ear from the middle ear in humans. It is like a stretched rubber sheet and sound vibrations make the eardrum vibrate. The eardrum sends these vibrations to the middle ear where the three bones, named malleus, incus and stapes, transmit and amplify these sound waves. From here the wave enters the inner ear which converts sound waves into electrical signals and send them to the brain. In this way, we are able to hear the sound around us.

Question 3.
Sound cannot travel in vacuum. Justify the statement with the help of an activity.
Answer:
Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it. Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands. Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. The sound becomes fainter as you suck the air. Now, remove the tumbler from your mouth. The sound becomes loud again. This shows that sound can travel in any medium (solid, liquid or gas) but not is vacuum.

Picture-Based Questions

Question 1.
Observe the diagram and answer the following questions.

a. Name the figure shown here.
b. Why are the different bowls tilled with different amounts of water?
Answer:
a. Jaltarang.
b. The different volumes of water create different heights of air column in the bowl. This helps to produce sounds of different frequencies and hence creates music.

Question 2.
Observe the following figures and identify which of these produces:
a. low pitched sound, and
b. high pitched sound.

Answer:
a. Drum produces low pitched sound,
b. Whistle produces high pitched sound.

These NCERT Solutions for Class 8 Science Chapter 12 Friction Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Friction NCERT Solutions for Class 8 Science Chapter 12

Class 8 Science Chapter 12 Friction Textbook Exercise Questions and Answers

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Question 1.
Fill in the blanks:
a. Friction opposes the ……………. between the surfaces in contact with each other.
b. Friction depends on the ……………. of surfaces.
c. Friction produces …………….
d. Sprinkling of powder on the carrom board ……………. friction.
e. Sliding friction is ……………. than the static friction.
Answer:
a. motion,
b. nature,
c. heat,
d. reduces,
e. less

Question 2.
Four children were asked to arrange forces due to rolling, static and sliding frictions in a ‘decreasing order’.
Their arrangements are given below. Choose the correct arrangement.
a. rolling, static, sliding
b. rolling, sliding, static
c. static, sliding, rolling
d. sliding, static, rolling
Answer:
c. static, sliding, rolling

Question 3.
Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in ‘increasing order’ will be:
a. wet marble floor, dry marble floor, newspaper and towel.
b. newspaper, towel, dry marble floor, wet marble floor.
c. towel, newspaper, dry marble floor, wet marble floor.
d. wet marble floor, dry marble floor, tow el, newspaper
Answer:
a. wet marble floor, dry marble floor, newspaper and towel.

Question 4.
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Answer:
When book slides down the desk, a frictional force acts between the book and the surface of the desk. The direction of the frictional force on the book is opposite to the direction of its motion and acts in an upward direction. It is shown in the diagram below:

Question 5.
You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer:
We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. It acts as a lubricant. Hence, it becomes difficult to walk on the soapy floor.

Question 6.
Explain why sportsmen use shoes with spikes.
Answer:
Shoes with spikes offer more friction because of more irregularities in the surface. This gives better grip to sportsmen while running.

Question 7.
Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer:
Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surfaces of box and floor becomes strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply a greater force than Iqbal.

Question 8.
Explain why the sliding friction is less than the static friction.
Answer:
When something is static, a greater force is required to break the interlocking between two surfaces. When something is in motion, the surfaces do not get enough time to interlock properly and a smaller force is required to keep the object in motion. Hence, sliding friction is less than static friction.

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Following two examples show that friction is both a friend and a foe:
Friction as a friend:

• We are able to walk because of friction.
• Friction between the tip of the pen and a paper allows us to write.

Friction as a foe:

• Tyres and soles of shoes wear out because of friction.
• Friction between the different parts of machines produces heat. This can damage the machines.

Question 10.
Explain why objects moving in fluids must have special shapes.
Answer:
When a body moves through a fluid, it experiences a force which tries to oppose its motion through the fluid. This opposing force is known as drag. This frictional force depends on the shape of the body. By giving the objects a special shape, called the streamlined shape, the force of friction acting on it can be minimised. Hence, it becomes easier for a body to move through the fluid.

NCERT Extended Learning Activities and Projects

Question 1.
What role does friction play in the sport of your choice? Collect some pictures of that sport in action where friction is either supporting it or opposing it. Display these pictures with proper captions on the bulletin board of your classroom.
Hint:
Supporting friction:
a. Cycling: The friction between the tyres and the road is necessary for the cycle to function.
b. Running: The friction between the athlete’s feet and the ground surface is necessary otherwise the runner would fall over.
c. Swimming: The friction in the water is required for the swimmer to be propelled forward.

Opposing friction:
a. Swimming: The friction makes it more difficult for the swimmer to swim very fast.
b. Bobsleigh: The aim is to go as fast as possible but the friction between the bottom of the sleigh and the ice slows it down.
c. Diving: The friction between the diver and the water causes wastage of kinetic energy and sound energy whereas the aim is to be as streamlined as possible.

Question 2.
Imagine that friction suddenly vanishes. How would the life be affected. List ten such situations.
Hint:
If frictional force were absent, we may face the following consequences:

• We cannot walk.
• We will not get a grip to hold things, and then we cannot eat, write, hold a pen or pencil, etc.
• Moving things cannot be stopped.
• Buildings cannot be constructed.
• We cannot fix a nail in the wall.
• We cannot stand properly without a grip.
• We would keep slipping.
• Nothing will be steady on ground. Things will not be at proper places because of absence of grip.
• Brakes in the vehicles will be useless.
• Finally, life would become miserable in the absence of friction.

Question 3.
Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations.
Hint:
Sports shoes have rough sole and spikes and the reason behind this is to give a better grip and traction on the grassy field. Also, if it is a rainy day, players (in football) can make better cuts with the cleats.

Question 4.
A toy to play with: Take an empty match box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown in the figure below. Fix the refill with two pins on the top of the tray as shown in figure. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out. Insert the tray in the outer cover of the matchbox.
Suspend the match box by the thread. Leave the thread loose. The match box will start falling down due to gravity. Tighten the thread now and observe what happens.
Explain your observation. Can you relate it to friction?

Hint:
Do it yourself.

Activity 1

Objective: To show that a rough surface exerts greater frictional force than a smooth surface.
Materials Required: A brick, a string, spring balance, polythene, jute bag.
Procedure:

• Tie a string around a brick.
• Pull the brick by a spring balance. You need to apply some force.
• Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.
• Now wrap a piece of polythene around the brick and repeat the activity and observe.
• Repeat the activity by wrapping a piece of jute bag around the brick and observe again.

Observation: In the first case, as the brick surface in contact with the ground is very rough, the brick experienced greater frictional force. But in the second case, when polythene is wrapped around the brick, the surface becomes smoother. So, in this case, the frictional force is less. The reading of spring balance is more than that of when brick was wrapped with polythene. Jute is also rough and exerts a larger frictional force.

Conclusion: A rough surface exerts a greater frictional force than a smooth surface.

Activity 2

Objective: To study that force of friction depends upon the nature of the two surfaces in contact.
Materials Required: A wooden board, a pencil cell, bricks or books, a piece of cloth and sand.
Procedure:

• Make an inclined plane on a smooth floor, or on a table. (You may use a wooden board supported by bricks or books.)
• Put a mark with a pen at any point A on the inclined plane.
• Now let a pencil cell roll down from this point.
• Observe how far does it move on the plane surface before coming to rest. Note down the distance.
• Now spread a piece of cloth over the table. Make sure that there are no wrinkle in the cloth. Try this activity again.
• Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.

Observation: The distance covered is minimum in the case when thin layer of sand is spread over the table. The distance covered is maximum on the plane surface. The distance covered by the pencil cell is different every time because of the difference in the amount of friction provided by the different surfaces.

Conclusion: Friction depends upon the nature of surfaces in contact.

Spring balance: It is a device used for measuring the force acting on an object. It consists of a coiled spring and a pointer moving on a graduated scale. When a force is applied, stretching of spring takes place. The reading on the scale indicated by the pointer gives the magnitude of the force.

Types of Friction:
i. Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
ii. Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box than to make a static box move.

iii. Rolling Friction: When an object rolls over a surface, the resistance produced to its motion is called rolling friction. Rolling friction is less than both the static and the sliding friction. It is always easy to roll than to slide a body over a surface.

Friction-A Necessary Evil: Friction is a necessary evil because it has both harmful and beneficial effects.
Benefits of friction:

• We can walk easily because ground offers friction. In less friction, walking becomes almost impossible. Therefore, friction is necessary even for a simple task like walking. It is difficult to move on a wet muddy track or wet marble floor. This is because these surfaces offer very little friction to the surface of the feet of a person.
• Writing with pen is possible because paper provides friction to the tip of pen.
• Writing with chalk on the blackboard is also possible because of friction. The blackboard surface rubs off some chalk particles which stick to it.
• Moving objects can be stopped because of friction.
• Friction between tyres of automobiles and road enables them to be started, stopped or change their direction of motion.
• Construction of building, fixing a nail on the wall, all require friction.

Evils of friction:

• Friction produces heat. Hence, when a machine is operated, heat generated due to friction causes much wastage of energy.
• Friction opposes the relative motion between objects.
• It causes wear and tear of materials whether they are shoes, ball bearings, screws, etc.
• Excessive friction may even result in unwanted fires, such as forest fires.

Ways to Increase Friction:
i. Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving better grip on these objects.
ii. A surface can be made rough with the help of sand paper in order to increase friction.
iii. Soles of shoes have grooves on them. Grooved soles give more friction to the ground which gives better grip when we walk. Shoes with worn out soles may be slippery.

iv. The tyres of vehicles have treads for better grip over the road. It provides friction to the surface of tyres. When treads are worn out, the tyres need to be replaced with new ones.
v. Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Ways to Reduce Friction:
i. Lubricating oil and grease are used in machines to reduce friction between the moving parts.

ii. Some machines use air cushion for reducing friction because oil is not ideal to be used in such machines.
iii. Graphite powder is used in some machines to reduce friction.
iv. Ball bearing converts sliding friction into rolling friction. You have read that rolling friction is less than sliding friction. Thus, ball bearings help in reducing friction.
v. Using wheels to drag load also covert sliding friction into rolling friction, there reducing its effect to a great extent.

Fluid Friction: When an object moves through a fluid (liquid or gas), the fluid creates friction. Friction created by a fluid is called fluid friction or drag. Force of friction is directly proportional to the density of a fluid. Hence, friction in oil will be greater than in water. Similarly, friction in water will be greater than that in air.

Streamlined Shape: A shape which is wide in middle and tapered at the ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fishes are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.

Class 8 Science Chapter 12 Friction Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is friction?
Answer:
The force which opposes the relative movement between two surfaces in contact is called friction.

Question 2.
Give an example to show that friction produces heat.
Answer:
Rubbing of palms together generates heat and makes us feel warm. This shows that friction produces heat.

Question 3.
Why are truck tyres treaded?
Answer:
Truck tyres are treaded to increase friction so as to provide a better grip on the ground.

Question 4.
Why is sliding replaced by rolling in most of the machines?
Answer:
Because rolling friction is smaller than sliding friction and facilitates smooth running of the machine.

Question 5.
Why do you think rolling friction is less than sliding friction?
Answer:
Rolling friction is less than sliding friction because in rolling, the surface area in contact is less than that in the sliding friction. Moreover, while rolling, the surfaces do not rub against each other properly.

Question 6.
Can we reduce friction to zero by using lubricants?
Answer:
No, it is not possible to eliminate friction entirely. No surface can have zero friction.

Question 7.
How can fluid friction be minimised?
Answer:
Fluid friction can be minimised by making the shape of the body streamlined.

Question 8.
Why is it difficult to walk on ice?
Answer:
It is difficult to walk on ice because ice offers very little friction between the feet and the ice surface.

Question 9.
Why does the force of friction exist?
Answer:
Friction is produced by the interlocking of the irregularities on the two surfaces in contact.

Question 10.
Why do the soles of shoes wear out?
Answer:
The soles of shoes wear out due to the friction between the soles and the ground.

Question 11.
Why are the soles of shoes grooved?
Answer:
Soles of shoes are grooved so as to provide better grip between the shoes and the ground.

Question 12.
Give two examples where rolling friction is utilised.
Answer:
Rolling friction is utilised by using ball bearings between the hub and the axle of a ceiling fan and in the bicycle.

Question 13.
On which factors does the friction depend?
Answer:
Friction depends on the nature and pressure of surfaces in contact.

Question 14.
What is meant by air resistance?
Answer:
The friction between a moving object and the air through which it moves is known as air resistance.

Question 15.
What is a streamlined shape?
Answer:
A shape which is wide in middle and tapered at the ends is called a streamlined shape.

Question 16.
Name the device used for measuring the force acting on an object.
Answer:
Spring balance.

Question 17.
Define viscous drag.
Answer:
The opposition offered to the motion of an object by a fluid is called viscous drag.

Short Answer Type Questions

Question 1.
It is easier to open a bottle cap by gripping it with a towel. Why?
Answer:
Rough surface of towel helps in increasing friction and thus provides a better grip for application of a longer force. The towel, thus, makes it easier to open the bottle cap.

Question 2.
Why is it difficult to drive on a wet road?
Answer:
It is very difficult to drive on a wet road because of less friction. A thin film of water is made over the surface of road. This reduces friction, because of which the driver finds it difficult to control the vehicle while driving on a wet road.

Question 3.
Why is friction said to be a necessary evil?
Answer:
Friction is all pervasive. Even a simple task, like writing on a page, cannot be done without friction. Walking on road will not be possible without friction. But friction causes wear and tear of materials, and wastage of energy. Thus, friction is beneficial as well as harmful. Flence, friction is called a necessary evil.

Question 4.
Boats and aeroplanes are given streamlined shape. Why?
Answer:
A shape which is wide in middle and tapered at the ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fishes are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.

Question 5.
Give two situations where it is desirable to increase friction.
Answer:
Two situations where it is desirable to increase friction are:
a. Tyres have designs and patterns with grooves on the surface to increase friction with the road. This prevents slipping of the tyres on the wet road.
b. Spikes are provided in the soles of shoes used by players and athletes to increase friction so that they get a firm grip on the ground and do not slip while running.

Question 6.
Give some advantages of friction.
Answer:
Advantages of friction are as follows:

• Friction between pen and paper enables us to write on the paper.
• It enables us to hold various things.
• Friction between our feet and the ground allows our movements like walking, jogging, etc.
• It holds the screw in a wood.
• It allows dishes to stand on table.
• Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping.

Question 7.
Give some disadvantages of friction.
Answer:
Disadvantages of friction are as follows:

• Friction opposes motion, so some of the energy is wasted in overcoming friction.
• It causes wear and tear of sole of shoes, for example, the sole of our shoes wears out after prolonged use because of friction with the rough road surface.
• The heat product by friction damages rubber parts of machines, and also results in energy loss.
• Friction causes moving objects to stop or slow down.

Question 8.
Why is it easier to tie a jute knot than a silk knot?
Answer:
The coefficient of friction of jute on jute is much more than that of silk on silk. Hence, two strands of a jute rope can ‘grip’ each other much more strongly than the two strands of a silk rope. This makes it easier to tie a knot in a jute rope as compared to that in a silk rope.

Question 9.
What is spring balance?
Answer:
It is a device used for measuring the force acting on an object. It contains a coiled spring. The spring gets stretched when a force is applied on it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of force.

Question 10.
How does lubrication reduce friction?
Answer:
Friction is due to roughness of surfaces. Any process that makes the contact surfaces smooth will reduce friction. Friction can be decreased by adding lubricants to the surfaces like oil, grease or graphite. The sliding surfaces then have a thin layer of the lubricants between them. The friction is then between the surfaces and the lubricant layer which is much less. For example, graphite is used as a lubricant in machines where the moving parts reach very high temperatures.
While playing carrom, fine talcum powder on the carrom board reduces friction.

Question 11.
Why is it not easy to move an object from its static position?
Answer:
When an object is at rest, it has better hold of the surface on which it is placed. In the static position, the irregularities of the surfaces are interlocked properly, due to which more force is required to overcome friction. Hence, it is not easy to move an object from its static position.

Question 12.
Why is it easier to pull luggage fitted with rollers?
Answer:
When a body rolls over the surface of another body, the friction is reduced. It is always easier to roll than to slide over another body. That’s why it is easier to pull luggage fitted with rollers.

Question 13.
Aman and Rohan are riding their bicycles on the same concrete road. Aman has new tyres on his bicycle while Rohan has tyres that are old and used. Aman suggested Rohan to change the tyres of his bicycle as soon as possible.
a. Which of the two boys is more likely to slip while moving through a patch of the rod which has lubricating oil spilled over it.
b. Why has Aman advised Rohan to change the tyres of his bicycle?
Answer:
a. Rohan.
b. This is because the worn out tyres do not offer friction and chances of skidding of vehicle increase.

Long Answer Type Questions

Question 1.
Discuss the different types of friction.
Answer:
Following are the three types of friction:
a. Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object move is called static friction.

b. Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.

c. Rolling Friction: When an object rolls over the surface of another object, the friction created is called rolling friction. Rolling friction is less than sliding friction.

Question 2.
Suggest some methods to increase friction.
Answer:
Some methods to increase friction are as follows:

• Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving a better grip on these objects.
• A surface can be made rough with the help of sandpaper, in order to increase friction.
• Soles of shoes are made rough to increase friction. This helps in providing a better grip on the surface while walking.
• Treads of tyres are deliberately made so that a good road grip can be achieved with the help of friction.
• Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Question 3.
Suggest some methods to reduce friction.
Answer:
Some methods to reduce friction are as follows:

• Lubricating oil and grease are used in machines to reduce friction between the moving parts.
• Some machines use air cushions for reducing friction because use of oil is not ideal in such machines.
• Graphite powder is used in some machines to reduce friction.
• Ball bearing converts sliding friction into rolling friction. Rolling friction is less than sliding friction. Thus, ball bearing helps in reducing friction.

Question 4.
Define fluid. Write the factors on which fluid friction depends.
Answer:
The gases and liquids are together called fluids. The friction exerted by fluids on an object is called fluid friction. The fluid friction is also called drag. The factors on which fluid friction depends are as follows:

• The fluid friction of an object depends on its speed with respect to the fluid.
• The fluid friction of an object also depends on the shape of the object.
• The fluid friction also depends on the nature and density of the fluid.

Picture-Based Questions

Question 1.
Observe the following figure and answer the questions.

a. What kind of friction do these things encounter.
b. What type of shape do they have?
c. How does this shape help than.
Answer:
a. Fluid friction.
b. They have a streamlined shape.
c. Streamlined shape reduces fluid friction and makes their movement easy.

Question 2.
Draw a diagram to show the surface irregularities.
Answer:

Question 3.
Observe the following figure and answer the questions.

a. What does the figure depict?
b. Why do soles of shoes wear out?
c. Is it possible to reduce friction up to zero?
Answer:
a. The figure shows grooves in the soles of shoes which increases friction.
b. Because of friction, the soles of shoes wear out.
c. No, it is not possible.

These NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Force and Pressure NCERT Solutions for Class 8 Science Chapter 11

Class 8 Science Chapter 11 Force and Pressure Textbook Exercise Questions and Answers

Page 142-143

Question 1.
Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer:
Examples of pushing force:

1. A heavy box at rest is pushed to move it from one room to another. This brings the box in motion.
2. A player pushes a football using his foot. This changes the state of motion and direction of the ball.

Examples of pulling force:

1. Rope is pulled to draw water from a well. This brings the bucket of water in motion.
2. A drawer is pulled to open it. This brings the drawer in motion.

Question 2.
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:
Examples of forces that cause a change in the shape of an object are as follows:

1. Squeezing a plastic bottle changes its shape.
2. Deformation of clay by pressing it between the hands.
3. Rolling a chapati from a ball of dough.

Question 3.
Fill in the blanks in the following statements:
a. To draw water from a well we have to ……………… at the rope.
b. A charged body ……………… an uncharged body towards it.
c. To move a loaded trolley we have to ……………… it.
d. The north pole of a magnet the ……………… north pole of another magnet.
Answer:
(a) pull,
(b) attracts,
(c) push or pull,
(d) repels

Question 4.
An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:
(muscular, contact, non-contact, gravity, friction, shape, attraction)
a. To stretch the bow, the archer applies a force that causes a change in its ………………
b. The force applied by the archer to stretch the bow is an example of ……………… force.
c. The type of force responsible for a change in the state of motion of the arrow is an example of a ……………… force.
d. While the arrow moves towards its target, the forces acting on it are due to ……………… and that due to ……………… of air.
Answer:
(a) shape,
(b) muscular,
(c) contact,
(d) gravity, friction

Question 5.
In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
a. Squeezing a piece of lemon between the fingers to extract its juice.
b. Taking out paste from a toothpaste tube.
c. A load suspended from a spring while its other end is on a hook fixed to a wall.
d. An athlete making a high jump to clear the bar at a certain height.
Answer:
a. In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force is exerted on the lemon by our fingers. As a result, the shape of the lemon gets changed.

b. We use our muscular force to take out paste from a toothpaste tube. The muscular force is exerted on the toothpaste tube by our fingers. As a result, the shape of the tube gets changed.

c. Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

d. An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.

Question 6.
A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?
Answer:
Force due to hammering changes the shape of the piece of iron.

Question 7.
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?
Answer:
Electrostatic force is responsible for the attraction between the balloon and the wall. The balloon rubbed with a synthetic cloth gets electrically charged and sticks to the wall. This is because a charged object attracts another charged or uncharged object.

Question 8.
Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
Answer:
In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. Both forces are acting in opposite directions and hence nullify the effect of each other. Due to this, there is no change in the state of motion of the bucket.

Question 9.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer:
The two forces acting on the rocket immediately after leaving the launching pad are:

• Upward force applied by the rocket engine.
• Downward gravitational force applied by the earth.

(Ignoring frictional force due to air resistance)
The upward force due to rocket engine exceeds the downward gravitational force which makes the rocket move upward.

Question 10.
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
a. pressure of water
b. gravity of the earth
c. shape of rubber bulb
d. atmospheric pressure
Answer:
d. atmospheric pressure

NCERT Extended Learning Activities and Projects

Question 1.
Make a 50 cm × 50 cm bed of dry sand about 10 cm in thickness. Make sure that its top surface is levelled. Take a wooden or a plastic stool. Cut two strips of graph paper each with a width of 1 cm. Paste them vertically on any leg of the stool – one at the bottom and the other from the top. Now gently put the stool on the sand bed with its legs resting on the sand. Increase the size of sand bed if required. Now put a load, say a school bag full of books, on the seat of the stool. Mark the level of sand on the graph strip. This would give you the depth, if any, to which the legs of stool sink in sand. Next, turn the stool upside down so that now it rests on its seat on the sand bed. Note the depth to which the stool sinks now. Next, put the same load on the stool and note the depth to which it sinks in the sand. Compare the pressure exerted by the stool in the two situations.
Hint:
The stool will sink to a greater depth in sand when kept with its legs resting on the sand. This is because the legs of stool have lesser surface area than its top and hence exert a greater pressure on sand.

Question 2.
Take a tumbler and fill it with water. Cover the mouth of the tumbler with a thick card similar to that of a postcard. Hold the tumbler with one hand while keeping the card pressed to its mouth with your other hand. Turn the tumbler upside down while keeping the card pressed to its mouth. Make sure that the tumbler is held vertical. Gently remove the hand pressing the card. What do you observe? Does the card get detached allowing the water to spill? With a little practice you will find that the card continues to hold water in the tumbler even after it is not supported by your hand. Also try this activity by using a piece of cloth to hold the tumbler in an upside down position.

Hint:
Do it yourself.

Question 3.
Take 4-5 plastic bottles of different shapes and sizes. Join them together with small pieces of glass or rubber tube as shown in figure below. Keep this arrangement on a level surface. Now pour water in any one of the bottles. Note whether the bottle in which water is poured gets filled first or all the bottles get filled up simultaneously. Note the level of water in all the bottles from time to time. Try to explain your observations.

Hint:
Water fills in bottles according to air pressure. The bottle which has lesser air pressure will fill first. This is because air pressure opposes water flow. However, at the end, water level in all the bottles will be the same.

Activity 1

Objective: To demonstrate the electrostatic force.
Materials Required: A plastic straw, thread and a sheet of paper.
Procedure:

• Take a plastic straw and cut it into nearly two equal pieces.
• Suspend one of the pieces from the edge of a table with the help of a piece of thread.
• Now hold the other piece of straw in your hand and rub its free end with a sheet of paper.
• Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other.
• Now, rub the free end of the suspended piece of straw with a sheet of paper. Again. bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw.

Observation: In the first case, the suspended straw was attracted towards the straw in the hand. In the second case, there is repulsion between the straws.

Conclusion: The force applied by a charged body on another charged or uncharged body is called electrostatic force. It is a non-contact force. Like charges repel while unlike charges attract each other.

c. Gravitational Force: Every object in the universe exerts a force on every other object. This force is called gravitational force. It is the gravitational force of various celestial bodies which keeps them in their position in the space. All things fall towards the earth because of gravitational force.

Pressure: Not only the magnitude of the force, but the area upon which it acts also affects the changes it may bring upon an object. The force acting upon a unit area is called pressure.
Pressure = \(\frac{\text { Force }}{\text { Area }}\)

The SI unit of pressure is Pascal (Pa) or N/m². Hence, the pressure exerted by an object depends upon its surface area. If the surface area is small, the amount of pressure applied is large for the same force, and vice-versa. Following examples illustrate this effect:

• It is easier to push a nail into a wooden door through its pointed end than through its blunt end because the smaller area of the pointed end helps in creating more pressure and it becomes easier to insert the nail.
• Shoulder straps of school bags are wide. A larger area helps in reducing the pressure on the shoulder, and makes it less painful to carry the bag.
• Camel’s feet are wide and have a larger area. Due to this, a camel can easily walk on sand.
• Eskimos wear ski-like footwear. Wider area of the footwear reduces pressure and thus prevents the Eskimo’s feet from sinking in the ice.

Pressure Exerted by Liquids and Gases:
i. Liquids and gases exert pressure on walls of the container in which they are kept. Containers for gases and liquids are hence, usually cylindrical in shape to equally distribute the pressure on all portions of the wall.

ii. The pressure exerted by a gas or a liquid on the bottom of its container, depends on the height of the column of gas or liquid in that container. Due to this, divers have to withstand a large pressure at the bottom of the sea.
iii. Liquids exert equal pressure at the same depth, while the pressure in liquids increases with depth.

Activity 2

Objective: To show that liquids exert equal pressure at the same depth.
Materials Required: Empty plastic bottle, a borer and water.
Procedure:

• Take an empty plastic bottle and drill four holes all around near its bottom. Make sure that the holes are at the same height from the bottom.
• Now fill the bottle with water and observe.

Observation: The water streams coming out of the holes fall at the same distance.

Conclusion: This shows that an equal pressure acts at the same depth in liquids.

Atmospheric Pressure: Our atmosphere is made up of different types of gases exert. Since gases exert pressure, the pressure exerted by air (present in atmosphere) is called atmosphere pressure. The weight of air in a column of the height of the atmosphere and area 10 cm x 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside. A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.

Activity 3

Objective: To show the effect of pressure due to air.
Material Required: A good quality rubber sucker.
Procedure:

• Take a good quality rubber sucker. It looks like a small rubber cup.
• Press it hard on a smooth, plane surface. It sticks to the surface very hard.
• Now try to pull it off the surface and observe.

Observation: The sucker cannot be pulled off the surface, if there were no air at all between the sucker and the surface.

Conclusion: This shows the effect of pressure due to air. It shows that the atmospheric pressure is very large.

Class 8 Science Chapter 11 Force and Pressure Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is force?
Answer:
A push or pull on an object is called force.

Question 2.
Define rest as a state of motion.
Answer:
When an object is not moving at all, i.e., has zero speed, it is said to be in the state of rest, e.g., a book lying on the table without any motion.

Question 3.
What is a contact force? Give examples.
Answer:
Contact force is a force which can produce its effect only by making a contact with the object, e.g., muscular force and frictional force.

Question 4.
What is a non-contact force?
Answer:
The non-contact force is a force which can produce its effect without making a contact with the object.

Question 5.
What is muscular force?
Answer:
It is the force applied by the muscles of humans or animals.

Question 6.
What is friction?
Answer:
When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction.

Question 7.
Which force stops the moving striker on a carrom-board?
Answer:
Frictional force.

Question 8.
What is electrostatic force?
Answer:
It is the force applied by a charged body on another charged or non-charged body.

Question 9.
Name any two types of non-contact forces.
Answer:
Magnetic force and gravitational force.

Question 10.
What is the relation between the direction of force of friction and the direction of motion?
Answer:
Direction of force of friction is always opposite to the direction of motion.

Question 11.
Which force enables the cars and buses to run on the roads?
Answer:
Cars and buses are able to run on roads because of the force of friction between the tyres and the road.

Question 12.
Does the force of gravitation exist between two objects on the earth?
Answer:
Yes, the force of gravitation exists between any two objects on the earth but it is very weak.

Question 13.
What is pressure?
Answer:
The force acting per unit area on an object is called pressure.

Question 14.
What is atmospheric pressure?
Answer:
The pressure exerted by atmospheric air is called atmospheric pressure.

Question 15.
Why don’t we feel atmospheric pressure?
Answer:
We don’t feel atmospheric pressure because pressure of air inside our body is equal to it and hence nullifies its effect.

Short Answer Type Questions

Question 1.
How do we feel force in our daily life?
Answer:
Many big or small actions make us feel the force. We have to push or pull many objects daily. A moving ball stops on its own. The ball changes the direction of its motion, when it is hit with a bat. Lifting, hitting, kicking, squeezing, etc., are all examples of force in daily life.

Question 2.
What happens when two people simultaneously exert equal forces in opposite directions on the same object?
Answer:
When two equal forces are exerted on an object from opposite sides in opposite direction, the forces get cancelled and there is no net force on the object. Such balanced forces cannot change the state of motion but the shape of the object can be changed.

Question 3.
What is the difference between contact and non-contact forces?
Answer:

Question 4.
Two tyres of a tractor are very wide and huge. Why?
Answer:
A wide tyre means a larger area. We know that pressure reduces when area increases. The wide tyres reduce the pressure exerted by the tractor. As a result, it becomes easier to drive a tractor on a muddy or a freshly ploughed surface.

Question 5.
It is easier to cut an apple with sharp end of knife rather than with its blunt end. Why?
Answer:
The sharp end of knife has much smaller area as compared to its blunt end. Smaller area provides more pressure for the same applied force. Hence, it becomes easier to cut an apple with sharp end of knife as compared to with its blunt end.

Question 6.
Why do some people suffer from nose bleeding at high altitudes?
Answer:
The atmospheric pressure is maximum on the surface of the earth. When we go to a higher altitude (say a high mountain), then the atmospheric pressure decreases. So, at high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than the outside pressure, therefore, some of the blood vessels in our body may burst and nose bleeding can occur at high altitudes.

Question 7.
Why do astronauts wear special suits in space?
Answer:
Space suits help astronauts in several ways. Space is extremely cold and filled with dangerous radiations. It has no air to breathe and no air pressure. A space suit protects astronauts from these extreme temperatures. Spacesuits also supply astronauts with oxygen to breathe while they are in the vacuum of space. They contain water to drink during spacewalks. They protect astronauts from being injured from the impact of small bits of space dust.

Question 8.
If the area of your head is 15cm × 15cm, how much air (in weight) would you be carrying on your head?
Answer:
The weight of air column of the height of the atmosphere and area 15cm × 15cm is equal to the weight of an object of mass 225kg (2250N). We have air inside our body, so we are not crushed under this weight and pressure of air is equal at both the sides.

Question 9.
What are the factors on which the effect of force depends?
Answer:
There are two factors on which the effect of force depends:

1. Magnitude: The strength of force is usually expressed by its magnitude.
2. Direction of force: We also have to specify the direction of force in which it acts. If the direction or magnitude of force changes, its effect also changes.

Question 10.
What are the effects of force on a moving object?
Answer:
A force can have the following effects on a moving object:

• When the applied force is in the same direction as direction of motion, there is an increase in the speed of the object.
• When the applied force is in the direction opposite to the direction of motion, there is a decrease in the speed of object. The object may also come to rest in this case.
• When the applied force is at some acute or obtuse angle to the direction of motion, there is a change in the direction of motion of the object.

Question 11.
Arpita was wearing a high heel shoe on a beach party. Her friends told her to wear flat shoes as she will be tired soon with high heels and will not feel comfortable.
a. Why does one feel tired with high heel shoes on a beach?
b. Give the SI unit of pressure.
Answer:
a. The high heel shoe exerts lots of force while we walk on the sand. When we walk with high heel shoes in sand, it will sink into the sand and will exert more pressure. Hence, one feels tired.
b. The SI unit of pressure is Pascal.

Long Answer Type Questions

Question 1.
Explain some characteristics of force.
Answer:
Following are some characteristics of force:

• At least two objects need to interact for force to come into play. Mutual interaction between two objects results in a force between the two objects.
• When two forces are applied on an object in the same direction, there is an add-on effect of both the forces.
• When two forces are applied in opposite directions, the difference between the two forces gives the value of the net force.
• Force is expressed in terms of its magnitude and the direction in which it is working. If there is a change in direction or magnitude of the force, there is a change in the effect of force.
• Generally, more than one force may be acting on an object at any given time.

Question 2.
Forces are due to an interaction between the objects. Explain.
Answer:
Suppose a man is standing behind a stationary car. The car will not move due to his presence. Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the direction of the applied force in this case. Hence, the man has to push the car to make it move.

From this example, we can infer that at least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the two objects.

Question 3.
What are the characteristics of the pressure exerted by liquids and gases?
Answer:
Liquids and gases exert pressure as follows:

• A fluid exerts pressure on the walls of the container. Containers for gases and liquids are usually cylindrical in shape to equally distribute the pressure on all portions of the wall.
• Pressure at the bottom of the container depends on the height of the column of gas or liquid filled in it. Due to this, divers have to withstand a large pressure at the bottom of the sea.
• A fluid exerts equal pressure at the same depth. The pressure increases with increase in depth.

Question 4.
What are contact forces? Explain different types of contact forces.
Answer:
Contact forces are the forces which come into play when two objects are in direct contact with each other. Following forces are the contact forces:
a. Muscular Force: The force which is exerted due to the action of muscles is called muscular force. Actions like bending, running, moving, etc., are the examples of the muscular force. It is a contact force because it comes into play only when there is a contact of the muscles with the object.

b. Friction: It is a force which opposes the relative motion between two surfaces in contact. It is also a contact force because it is exerted when two surfaces come in contact with each other. Halting of a moving ball on its own is an example of force of friction which acts between the ball and surface in this case.

Question 5.
Explain the different types of non-contact forces.
Answer:
a. Magnetic Force: The force exerted by any magnetic object is called magnetic force. We know that like magnetic poles always repel each other, that is, they push each other away. Also, opposite magnetic poles always attract each other, that is, they pull each other towards themselves.

b. Electrostatic Force: The force exerted by a charged particle on another charged or uncharged particle is called electrostatic force. We know that like charges always repel or push each other away. Similarly, opposite charges always attract or pull each other towards themselves.

c. Gravitational Force: It is an attractive force that acts on all objects. It is the force by which the earth pulls the objects towards itself. Gravity is a property exhibited by every object present in the universe and not only the earth. Hence, all the planets, the moons and even the sun, all have a gravitational force of their own.

Question 6.
Explain with the help of an activity the pressure exerted by a liquid at the bottom of the container depends on the height of its column.
Answer:
Activity:
a. Take a plastic pipe open at both ends. To one end, tie a thin rubber sheet.
b. Ask one of your friends to pour some water in the pipe. The rubber sheet bulges out when the water column in the pipe rises.
c. Note the height of the w’ater column in the pipe.
d. Pour some more water. Again observe the bulge in the rubber sheet and the height of the water column in the pipe.

Conclusion: The water poured in the pipe begins to apply pressure on the thin rubber sheet, due to which it begins to bulge. The bulge increases with increase in the height of water column. This shows that pressure depends on the height of liquid column.

Picture-Based Questions

Question 1.
Observe the following figure and answer the questions.

a. What are these two boys in the figure doing?
b. What is the direction of forces applied by these two men?
c. What will be the net force in this case?
Answer:
a. These two boys are applying the force on a block.
b. Both of them are applying the force in opposite directions.
c. The difference of two forces will be the net force in this case.

Question 2.
Observe the following figure and answer the questions.

a. What is the man doing?
b. Why do such men place a round piece of cloth on their head?
Answer:
a. This porter is carrying heavy loads on his head.
b. They use the round piece of doth on their head because it increases the surface area on which the force will act and this reduces pressure on their head.

Question 3.
Observe the following figure and answer the questions.

a. What are the bullocks doing?
b. What type of force is applied by the bullocks?
c. Is it a contact or a non-contact force?
Answer:
a. Bullocks are drawing the heavy loads.
b. Muscular force is applied by the bullocks.
c. It is a contact force.

These NCERT Solutions for Class 8 Science Chapter 10 Reaching the Age of Adolescence Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Reaching the Age of Adolescence NCERT Solutions for Class 8 Science Chapter 10

Class 8 Science Chapter 10 Reaching the Age of Adolescence Textbook Exercise Questions and Answers

Page 124-125

Question 1.
What is the term used for chemical secretions of endocrine glands responsible for changes taking place in the body?
Answer:
The chemical substances secreted by the endocrine glands are called hormones.
Hormones are responsible for the various changes taking place in the body.

Question 2.
Define adolescence.
Answer:
The period of life when many changes are taking place in the body leading to reproductive maturity, is called adolescence. It begins around the age of 11 years and lasts upto 18 or 19 years.

Question 3.
What is menstruation? Explain.
Answer:
In females, one egg is released by either of the ovaries each month. After that, the uterine walls develop a thickening to support the foetus and prepare for a possible pregnancy. If fertilisation does not take place, the egg and the thickening of uterus walls are discarded in small fragments. This results in bleeding through the vagina for some days. The bleeding (through vagina) at the end of menstrual cycle is called menstruation.

Question 4.
List changes in the body that take place at puberty.
Answer:
Following changes take place in the body at the time of puberty:

• Sudden increase in height and weight.
• Broadening of shoulders and widening of chest in boys. In girls, the region below waist becomes wider.
• In boys, under the influence of hormones, the larynx becomes prominent and the vocal cords become longer and thicker. These changes make the voice hoarse.
• Appearance of hair in underarms, face, hands, legs and pubic region.
• Appearance of acne as a result of excessive secretion of oil glands in the skin.
• Testes grow and start producing sperms in males, whereas in females, the ovary enlarges and starts producing mature eggs.

Question 5.
Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.
Answer:
The table below shows endocrine glands and the hormones secreted by them:

Question 6.
What are sex hormones? Why are they named so? State their function.
Answer:
Hormones which influence the functions of gonads and facilitate the appearance of secondary sexual characters are called sex hormones. Testosterone and estrogen are the main sex hormones.
i. Testosterone: In males, the testes produce the male sex hormone called testosterone. This hormone helps in the development and maintenance of the primary and secondary sexual characters such as growth of beard, development of reproductive organs, production of sperms, etc.

ii. Estrogen: In females, the ovaries secrete estrogen and progesterone which are responsible for the development of primary and secondary sexual characters such as the enlargement of breasts, development of female reproductive organs, etc.

Question 7.
Choose the correct option:
a. Adolescents should be careful about what they eat, because
i. proper diet develops their brains
ii. proper diet is needed for the rapid growth taking place in their body
iii. adolescents feel hungry all the time
iv. taste buds are well developed in teenagers
Answer:
(ii) proper diet is needed for the rapid growth taking place in their body.

b. Reproductive age in women starts when their
i. menstruation starts
ii. breasts start developing
iii. body weight increases
iv. height increases
Answer:
(i) menstruation starts

c. The right meal for adolescents consists of
i. chips, noodles, coke
ii. chapati, dal, vegetables
iii. rice, noodles and burger
iv. vegetable cutlets, chips and lemon drink
Answer:
(ii) chapati, dal, vegetables

Question 8.
Write notes on-
a. Adam’s apple
b. Secondary sexual characters
c. Sex determination in the unborn baby
Answer:
a. Adam’s apple: At puberty, the voice box or the larynx begins to grow. Boys develop larger voice boxes. The increase in the size of voice box in boys is seen as a prominence in the neck. This protrusion is called Adam’ apple.

b. Secondary sexual characters: These are the characters which distinguish a male from a female are called secondary sexual characters. Some examples are given below:

• In males, the growth of hair is seen on face and body.
• Hair growth in the pubic region, under the arms, etc., is seen in both males and females.
• In boys, voice becomes deeper.
• Increase in weight.
• Shoulders become broad and chest gets widened in males. The hips become wider in females.
• The development of breasts takes place in females.
• Mood swings are seen in males as well as females. Mental and emotional maturity is attained by both the sexes. Brain becomes active and has a larger capability of learning.

c. Sex determination in the unborn baby: Gametes in humans have 23 chromosomes, i.e., half the total number of chromosomes in somatic cells. So, a sperm may have an X or a Y chromosome as 23rd chromosome. But, an ovum has only X chromosome as 23rd chromosome. If a sperm with X chromosome fertilises the egg, the zygote thus formed will lead to the birth of a girl child. If a sperm with Y chromosome fertilises the egg, the zygote thus formed will lead to – the birth of a male child.

Question 9.
Word game: Use the clues to work out the words.
Across
3. Protruding voice box in boys
4. Glands without ducts
7. Endocrine gland attached to brain
8. Secretion of endocrine glands
9. Pancreatic hormone
10. Female hormone

Down
1. Male hormone
2. Secretes thyroxine
3. Another term for teenage
5. Hormone reaches here through bloodstream
6. Voice box
7. Term for changes at adolescence

Answer:
Across
3. Adam’s apple
4. Endocrine
7. Pituitary
8. Hormone
9. Insulin
10. Estrogen

Down
1. Testosterone
2. Thyroid
3. Adolescence
5. Target site
6. Larynx
7. Puberty

Question 10.
The table below shows the data on likely heights of boys and girls as they grow in age. Draw graphs showing height and age for both boys and girls on the same graph paper. What conclusions can be drawn from these graphs?

Answer:

We conclude that initially, girls grow faster than boys but, by about 18 years of age both reach their maximum height. Boys are, in general, taller than girls. The rate of growth in height varies in different individuals.

NCERT Extended Learning Activities and Projects

Question 1.
Find out from your elder relatives about their awareness of the legal status of early marriage. You yourself may get information on it from your teacher, parents, a doctor or the internet. Write a two-minute speech explaining why early marriage is not good for the couple.
Hint:
Early marriage has a great impact on the body of a woman. Early marriages lead to early pregnancies and because of these pregnancies the life of a girl suffers danger. Marriage needs lots of commitment, understanding and the capability to shoulder numerous responsibilities which cannot be expected from two very young people. Maturity is something that is directly related to age. It may vary from person to person but there’s no doubt that a young couple will have a sense of immaturity. Moreover, getting married in early age means that the couples do not even have financial stability which leads to the collapse of such family. In addition, this also creates obstruction for their studies. Hence, early marriages are not good at all.

Question 2.
Collect newspaper cuttings and information in magazines about HIV/ AIDS. Write a one page article of 15 to 20 sentences on HIV/AIDS.
Hint:
HIV stands for Human Immunodeficiency Virus. It harms the immune system by destroying the white blood cells that fight infection. This puts a person at risk of serious infections and certain cancers. AIDS stands for Acquired Immunodeficiency Syndrome. It is the final stage of infection with HIV. Not everyone with HIV develops AIDS. HIV most often spreads through unprotected sex with an infected person. It may also spread by sharing drug needles or through contact with the blood of an infected person. Women can give it to their babies during pregnancy or childbirth. The first signs of HIV infection may be swollen glands and flu-like symptoms. These may come and go within two to four weeks. Severe symptoms may not appear until months or years. A blood test can tell if you have an HIV infection. Your health care provider can do the test, or you can use a home testing kit. There is no cure, but there are many medicines that fight HIV infection and lower the risk of infecting others. People who get early treatment can live with the disease for a longer time.

Question 3.
In our country, according to 2011 census, there are 940 adolescent females for every 1000 males. Find out
a. the concerns of the community regarding this low ratio. Remember that the chance of having a boy or a girl is equal.
b. what amniocentesis is and how useful this technique is. Why is its use for identification of sex of the unborn child banned in India?
Hint:
a. The concerns of the community regarding the low ratio are as follows:

• Social imbalance: The decrease in sex ratio would result in social imbalance. In a society the number of male and female should be equal because unequal number would result in social imbalance.
• Marriage squeeze: With the declining number of sex ratio, the institution of marriage would be worst affected because there will be no girl for marriage.
• Immoral trafficking: As the sex ratio plunges down, people would resort to immoral trafficking where girls would be sold and bought for marriage. Their status would be degraded.
• Sexual offences against women would rise with decline in sex ratio. There would also be a rise in sexually transmitted diseases.

b. Amniocentesis is a diagnostic test usually done during pregnancy. This test involves removing small amount of amniotic fluid (the fluid surrounding a developing foetus) from the uterus. The amniotic fluid can be tested to determine if the fetus has certain kinds of birth defects.

Usage of this test is banned because of illegal practice of using it to determine the sex of the child and killing the unborn girl child (female infanticide).

Question 4.
Put your ideas together and write a short note on the importance of knowing facts about reproduction.
Hint:
By knowing the facts about reproduction, humans can either plan to have children or avoid unwanted pregnancies. It helps to remove social superstitions and be better aware of ourselves. It can also reduce the chances and spread of sexually transmitted diseases. It also helps to maintain hygiene and health. Hence, it is important.

Activity 1

Objective: To study and compare the gain in height of boys and girls at different ages.
Procedure: Use the data given below to draw a graph. Take age on the X-axis and present growth in height on the Y-axis. Highlight the point representing your age on the graph. Find out the percentage of height you have already reached. Calculate the height you might eventually reach.

Observation:

Suppose your present height is 120 cm and the point of your age is 14 years which touches the 93% mark on height axis.
So,
Full height = \(\frac{\text { Present height }(\mathrm{cm})}{\% \text { of full height at this age }}\) × 100
= \(\frac{120 \mathrm{~cm}}{93}\) × 100
= 129 cm
Hence, the height you may eventually reach is 129 cm.

Conclusion: It is evident from the graph that initially, girls grow faster than boys but by about 18 years of age, both reach their maximum height.

Secondary Sexual Characters: Characters which help in distinguishing a male from a female are called secondary sexual characters. The marked difference between the males and females of a species because of apparent physical traits is called sexual dimorphism.

• Secondary sexual characters in boys: Facial hair, deep voice and hair on chest.
• Secondary sexual characters in girls: Enlargement of breasts and high-pitched voice.
• Some common characters in girls and boys: Hair under armpits and in pubic region.

Hormones: Hormones are the chemical substances secreted by the endocrine glands for proper functioning and controlled changes taking place during adolescence in the human body. Endocrine glands are also known as ductless glands. These glands release hormones directly into the bloodstream to reach a particular body part known as the target site.

• In boys, testes secrete the hormone testosterone. This hormone is responsible for the secondary sexual characters in males, like growth of facial hair, etc.
• In girls, the ovaries secrete the hormone estrogen. This is responsible for the secondary sexual characters in females, e.g., development of breasts, etc.

Hormonal control: The production of sex hormones (testosterone from testes and estrogen from ovaries) is under the control of another hormone secreted from an endocrine gland called pituitary gland. The pituitary secretes many hormones, one of which stimulates the maturation of ova in the ovaries and sperms in the testes.

Reproductive Phase of Life in Humans: The reproductive phase begins with the onset of puberty and it happens at around the same age in boys and girls. But reproductive phase in men lasts much longer than in females. Reproductive phase in women lasts till about 50 years of age, while in men it lasts even beyond 60 years of age.
i. Menstruation: In females, one egg is released by either of the ovaries every month. After that, the uterus develops a thickening to support a foetus to prepare for a possible pregnancy. If fertilisation does not take place, the egg and the thickening in uterus walls are discarded in small fragments. This results in bleeding through the vagina for some days. The bleeding (through vagina) at the end of menstrual cycle is called menstruation.

ii. Menstrual Cycle: The cycle of events which begins from the maturation and release of an egg and ends with shedding off of the uterine lining along with the egg is called menstrual cycle. A menstrual cycle is usually of 28 to 30 days.

iii. Menarche: The first menstrual bleeding in a woman’s life is called menarche. Menarche marks the beginning of puberty in girls.

iv. Menopause: Cessation of menstrual cycle is called menopause. This happens at around 45-50 years of age. Menopause marks the end of reproductive phase in females.

Sex determination: Gender of a child is determined by the combination of chromosomes in the zygote. A normal human cell has 46 chromosomes, i.e., 23 pairs of chromosomes. Out of them, 22 pairs of chromosomes are identical. But chromosomes in the 23rd pair may be identical or different.

There are two types of chromosomes in 23rd pair. They are called X and Y chromosomes (together called sex chromosomes). The 23rd pair in a male has XY combination, while in a female it has XX combination.

Gametes in humans have 23 chromosomes, i. e., half the total number of chromosomes in usual body cells. So, a sperm may have an X or a Y chromosome as 23rd chromosome. But, an ovum always has an X chromosome as 23rd chromosome.
i. If a sperm with X chromosome fertilises the egg, the zygote thus formed will have an XX combination and will lead to the birth of a girl child.
ii. If a sperm with Y chromosome fertilises the egg, the zygote thus formed will have an XY combination and will lead to the birth of a male child.

harmones other than sex Harmones:

Role of Hormones in Completing the Life Cycle of Insects and Frogs: Insect hormones control the metamorphosis of larva into adult. Thyroxine hormone controls the metamorphosis of tadpole larva into frog. If a frog lays eggs in iodine deficient water, the tadpoles will not develop into frogs and will perish.

Activity 2

Objective: To study the importance of consuming iodised salt.
Procedure: Collect information from magazines or from doctors and prepare a note on the importance of consuming iodised salt. You can also look for this information on the internet.

Observation: Iodised salt is important for the proper functioning of thyroid gland. This gland produces a hormone called thyroxine which regulates protein, fat and carbohydrate metabolism in the body. Under Secretion of this hormone causes cretinism in children (which leads to feeble mindedness) and goitre in adults (in which neck becomes swollen). Iodine is required for the production of this hormone. Hence, consuming iodised salt with food improves metabolism.

Reproductive Health:
i. Balanced Diet: Rapid growth during adolescence necessitates balanced diet which contains all the nutrients in adequate amount. A teenager should take sufficient amount of carbohydrates, proteins, fats, vitamins and minerals in food. One should avoid eating fried food and junk food. Girls need to take a lot of iron-rich food because iron is necessary for the formation of RBCs in blood.

ii. Personal Hygiene: You have read that sweat and sebaceous glands show increased activity during this period. If proper hygiene is not maintained, it can lead to many skin diseases in adolescents. Girls need to take special care of hygiene during menstrual period.

iii. Physical Exercise: Physical exercise is helpful not only in making a strong body but also in properly utilising the energy. A good workout ensures that all your systems are working in perfect condition.

iv. Say ‘NO’ to Drugs: The menace of drugs is spoiling many lives. During adolescence, one can easily get influenced and carried away by negative stimulations. You need to have a strong will power to say ‘No’ to drugs. Drugs are addictive and they harm the body in the long run. They ruin health and happiness.

v. HIV (Human Immunodeficiency Virus): This is a virus which causes AIDS (Acquired Immunodeficiency Syndrome). Till date, there is no cure for this disease. Hence, prevention is the only way to keep away from this dangerous disease. AIDS can spread through sexual contact, through infected needles and from a mother to her unborn child. This can also be transmitted to an infant through mother’s milk.

Class 8 Science Chapter 10 Reaching the Age of Adolescence Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is puberty?
Answer:
The process of changes taking place in the body during adolescence to attain reproductive maturity is called puberty.

Question 2.
What indicates that a child is becoming an adult?
Answer:
The changes taking place in the body indicate that a child is becoming an adult.

Question 3.
How many pairs of chromosomes are found in the nuclei of the usual cells of human both’?
Answer:
23 pairs.

Question 4.
Name the disease caused by the lack of thyroxine hormone.
Answer:
Goitre.

Question 5.
Name the primary sex organs of human male and female.
Answer:
Testes in mates and ovaries in females.

Question 6.
At what age does the growth in human body become noticeable?
Answer:
After crossing the age of 10 or 11 years, a sudden spurt in the growth of human body becomes noticeable.

Question 7.
Is mother responsible for the sex determination of the baby?
Answer:
No, father is responsible for the sex determination of the baby as being heterogametic.

Question 8.
Do boys and girls undergo same physical changes during adolescence?
Answer:
The boys and girls undergo different physical changes during adolescence because of sexual dimorphism in humans.

Question 9.
What are the common secondary sexual characters in boys and girls of teenage?
Answer:
Hair under armpits, hair in pubic region.

Question 10.
Which gland is known as the master gland?
Answer:
Pituitary gland.

Question 11.
Which gland is responsible for the fatness in the body?
Answer:
Thyroid gland.

Question 12.
Which hormone causes sweating under fear?
Answer:
Adrenalin.

Question 13.
Do all the eggs carry similar set of chromosomes?
Answer:
Yes, all eggs carry the X chromosome.

Question 14.
What are twins?
Answer:
Twins are two offsprings produced by the same pregnancy. These can be identical or non-identical.

Short Answer Type Questions

Question 1.
What is sex chromosome? Explain briefly.
Answer:
The 23rd pair of chromosome in humans is called sex chromosome. It is of two types, i.e., X chromosome and Y chromosome. While XX combination in the 23rd pair results in the birth of a girl child, XY combination leads to the birth of a male child. Since these chromosomes are responsible for the determination of sex of the child, they are called sex chromosomes.

Question 2.
What is target site?
Answer:
Endocrine glands release hormones into the bloodstream to reach a particular body part which is called the target site. It is the site of body where the hormone produces its effects.

Question 3.
Explain in brief the development of sex organs in males and females.
Answer:
At puberty, male sex organs like testes and penis develop completely. The testes also begin to produce sperms. In girls, the ovaries enlarge and eggs begin to mature. Ovaries also start releasing mature eggs every month.

Question 4.
Write a short note on goitre.
Answer:
Goitre is a metabolic disease which happens due to iodine deficiency in the body. Iodine deficiency results in less production of thyroxine. This may result in goitre which is manifested by a permanent swelling in the neck.

Question 5.
Explain the cause of changes taking place in human body during puberty?
Answer:
There is a pituitary gland present in the body which controls the production of the hormones in both males and females. The hormones are the chemical substances released in the blood by endocrine glands like ovaries, testes or pituitary gland. The hormones secreted by pituitary stimulate testes and ovaries to secrete the male and female hormones, respectively. These hormones are responsible for the changes taking place in human body during puberty.

Question 6.
How can young boys and girls take care of their personal hygiene?
Answer:

• Everyone should take a bath daily because increased activity of sweat and oil glands makes the body unclean and prone to microbial growth.
• Girls should take special care of cleanliness during the time of menstrual flow.

Question 7.
Write the name and function of one male and one female sex hormone.
Answer:

• Male sex hormone is testosterone which is produced by testes. It produces secondary sexual characters in males and supports the production of sperms.
• Female sex hormone is estrogen which is produced by ovaries. It produces secondary sexual characters in females and supports the maturation of eggs.

Question 8.
Explain the role of hormones in completing the life cycle of frogs and insects.
Answer:
The life cycle of insects involves four different stages, namely, egg, larva, pupa and adult. The process of conversion of larva into adult through drastic changes is called metamorphosis. In insects, metamorphosis is controlled by insect hormones in the same way as is controlled by thyroxine hormone in frogs. Tadpole becomes a frog under the action of thyroxine hormone produced by thyroid. Since the production of thyroxine requires iodine, the tadpoles cannot become adults if they grew in iodine-deficient water.

Question 9.
What kind of secondary sexual characters are developed in males?
Answer:
The features in male are development of beard and moustaches. There is facial hair growth and development of deep voice. Shoulders become broad and height is increased.

Question 10.
What are the secondary sexual characters in females?
Answer:
There is a development of mammary glands or milk producing glands, development of a shrill voice, starting of menstrual cycle and widening of pelvic area.

Question 11.
What is the relation between adolescence and puberty?
Answer:
Adolescence is the period of life when a body undergoes changes leading to reproductive maturity. These changes mark the onset of puberty. Hence, adolescence is the period of life which witnesses puberty.

Question 12.
Write a short note on changes in body shape during puberty.
Answer:
There is change in body shape during puberty which includes broader shoulders and wider chests in boys and in girls, the region below the waist becomes wider.

Question 13.
How are general growth and sexual maturation different from each other?
Answer:
General growth refers to different types of developmental processes in the body like increase in height, weight gain, changes in shape and size of the body, etc. However, sexual maturation is specific to the changes reflected at puberty, like cracking of voice, new hair patterns, development of breast in females, etc.

Question 14.
The onset of puberty is controlled by hormones. Justify this statement.
Answer:
Hormones from pituitary gland stimulate the testes and ovaries to release testosterone in males and estrogen in females respectively these hormones are released in the bloodstream and reach their target sites, that is, the particular body parts where they have to show their effects. The target sites respond to the hormone and stimulate the changes in the body at onset of puberty. Thus, we can say that the onset of puberty is controlled by hormones.

Question 15.
Explain the function of pituitary gland.
Answer:
The production of all hormones is under the control of pituitary gland. The pituitary gland is termed as the ‘Master Gland’ because it directs other organs and endocrine glands, such as the adrenal glands and thyroid glands, to suppress or induce hormone production. It involves the following functions:

• Growth hormone production
• Production of hormones that act on other endocrine glands
• Production of hormones that act on the muscles and the kidneys
• Regulation of functions of endocrine glands.
• Storage of hormones produced by the hypothalamus

Long Answer Type Questions

Question 1.
Explain the menstruation and menopause.
Answer:
In girls, with the onset of puberty, the ovaries start releasing mature eggs. Either of the two ovaries produce one mature ovum after every four weeks. The process by which a mature ovum is released from an ovary is called ovulation. During this period, the wall of the uterus becomes thick. This is a natural preparation to receive the fertilised egg, in case fertilisation occurs.

If fertilisation does not occur, the released egg, and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding called menstrual flow, and the process is called menstruation. The menstrual flow lasts for 2-7 days. Menstruation occurs once in about 28-30 days. At 45 to 50 years of age, the menstruation cycle stops. The stoppage of menstrual cycle and other associated events in the body of a female is called menopause.

Question 2.
Describe the sex determination of a baby in humans.
Answer:
We know that there are 23 pairs of chromosomes present inside the nucleus of every human cell. These chromosomes have a thread-like structure and they always occur in pairs. Out of the 23 pairs of chromosomes, the last pair is of sex chromosomes. X and Y are the two sex chromosomes. Females have two X chromosomes and males have one X and one Y chromosome. The gametes contain only one set of chromosomes (23 chromosomes), that is, either an X or a Y chromosome. The egg of the female always contains an X chromosome. However, the sperm may contain either of the X and Y chromosomes. So, if the sperm that contains a Y chromosome fertilises the egg, the zygote develops into a male. On the other hand, if the sperm that contains an X chromosome fertilises the egg, the zygote develops into a female. Hence, the sex of the child depends upon the chromosomes of the father and not the mother.

Question 3.
What should be done to maintain the physical and mental health of adolescents?
Answer:
During adolescence, there is rapid mental and physical growth. Therefore, taking care of physical and mental health at this stage is of utmost importance.
For proper physical health, one needs:

a. A balanced diet: An adolescent should consume a balanced diet containing the right amount of proteins, carbohydrates, fats, vitamins and minerals. The diet should contain adequate amount of cereals for carbohydrates; meat, milk, nuts and pulses for proteins; controlled amounts of butter/ghee/ oils and sugars for energy; and fruits and vegetables for protection against diseases. Fast food which is tasty but does not have adequate nutrition, such as chips or aerated drinks, should not be used as substitute for meals.

b. Personal hygiene: Due to increased activity of sweat and sebaceous glands, proper personal hygiene is very important for adolescents. Taking a bath every day and cleaning all parts of the body is essential, otherwise body odour and bacterial infection may occur. Girls should be specially careful about hygiene during menstruation.

c. Physical exercise: Walking, jogging, aerobics, outdoor games, etc., are good for the growing adolescent body.

Question 4.
Make a chart of various endocrine glands, their hormones and one function of each gland.
Answer:

Picture-Based Questions

Question 1.
Observe the diagram given below and answer the following questions:

a. Name the glands labelled A, B, C and D.
b. Name the hormones produced by each of the glands E and F.
c. What is the function of the gland labelled D.
Answer:
a. A – Pituitary gland
B – Thyroid gland
C – Adrenal gland
D – Pancreas

b. E – Estrogen
F – Testosterone

c. D is pancreas which releases insulin. Insulin maintains the blood sugar levels.

Question 2.
Observe the figure and answer the following questions:

a. Name the part shown here.
b. Is it present in males or in females?
Answer:
a. Adam’s apple.
b. Males.

These NCERT Solutions for Class 8 Science Chapter 9 Reproduction in Animals Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Reproduction in Animals NCERT Solutions for Class 8 Science Chapter 9

Class 8 Science Chapter 9 Reproduction in Animals Textbook Exercise Questions and Answers

Page 110-111

Question 1.
Explain the importance of reproduction in organisms.
Answer:
Reproduction is very important for the organisms. The organisms reproduce to generate young ones like them. It helps to maintain their existence generation after generation. If reproduction does not take place, no living being will survive on the earth. Had the process of reproduction not been there, all the plants and animals would have become extinct. Secondly, special characters of an organism are carried over to its next generation through reproduction only.

Question 2.
Describe the process of fertilisation in human beings.
Answer:
The mode of reproduction in human beings is sexual reproduction. Ovaries in females produce ova while the testes in males produce sperms. Millions of sperms are ejected inside the female reproductive tract where one of them fuses with the ovum and forms zygote. This process is known as internal fertilisation. The zygote, then, begins to develop into an embryo which attaches to the female uterus wall.

Question 3.
Choose the most appropriate answer:
a. Internal fertilisation occurs
(i) in female body
(ii) outside female body
(iii) in male body
(iv) outside male body
Answer:
(i) in female body

b. A tadpole develops into an adult frog by the process of
(i) fertilisation
(ii) metamorphosis
(iii) embedding
(iv) budding
Answer:
(ii) metamorphosis

c. The number of nuclei present in a zygote is
(i) none
(ii) one
(iii) two
(iv) four
Answer:
(ii) one

Question 4.
Indicate whether the following statements are True (T) or False (F):
a. Oviparous animals give birth to young ones. ( )
b. Each sperm is a single cell. ( )
c. External fertilisation takes place in frog. ( )
d. A new human individual develops from a cell called gamete. ( )
e. Egg laid after fertilisation is made up of a single cell. ( )
f. Amoeba reproduces by budding. ( )
g. Fertilisation is necessary even in asexual reproduction. ( )
h. Binary fission is a method of asexual reproduction. ( )
i. A zygote is formed as a result of fertilisation. ( )
j. An embryo is made up of a single cell. ( )
Answer:
a. False
b. True
c. True
d. False
e. False
f. False
g. False
h. True
i. True
j. False

Question 5.
Give two differences between a zygote and a foetus.
Answer:

Question 6.
Define asexual reproduction. Describe two methods of asexual reproduction in animals.
Answer:
When a single parent is involved in reproduction and gametes are not formed, it is called asexual reproduction. Two methods of asexual reproduction are as follows:

i. Budding: Budding involves the formation of a new individual from the bulges or outgrowths, known as buds, formed on the parent body. This method of reproduction is common in Hydra. In Hydra, the cells divide rapidly at a specific site and develops into an outgrowth, called the bud. These buds, while being attached to the parent plant, develop into smaller individuals. When these individuals become mature enough, they detach from the parent’s body and become independent individuals.

ii. Binary Fission: It is a type of asexual reproduction in which a single cell divides into two halves. Organisms that reproduce through binary fission are bacteria and Amoeba. In Ainoeba, the division of cells can take place in any plane. It involves the division of its nucleus into two nuclei, which is followed by the division of the cytoplasm into two halves. Each half of the body receives a nucleus.

Question 7.
In which female reproductive organ does the embryo get embedded?
Answer:
The embryo gets embedded in the wall of the uterus. The embryo, while it is still attached to the uterus, gradually develops the various body parts such as hands, legs, head, eyes, etc. The embryo at this stage is called a foetus.

Question 8.
What is metamorphosis? Give examples.
Answer:
In case of indirect development, the transformation of the young ones into an adult through drastic changes is called metamorphosis. Larva of butterfly undergoes metamorphosis to become a butterfly. A tadpole undergoes metamorphosis to become a frog.

The tadpole that emerges from the egg contains gills, a tail and a small circular mouth. It can swim freely in water. The tadpole grows and undergoes abrupt changes in its structure and develops into a mature frog. A tadpole’s metamorphosis begins with the development of limbs, lung development and finally the absorption of the tail by the body.

Question 9.
Differentiate between internal fertilisation and external fertilisation.
Answer:

Question 10.
Complete the crossword puzzle using the hints given below.
Across
1. The process of the fusion of the gametes.
6. The type of fertilisation in hen.
7. The term used for bulges observed on the sides of the body of Hydra.
8. Eggs are produced here.
Down
2. Sperms are produced in these male reproductive organs.
3. Another term for in-vitro fertilisation.
4. These animals lay eggs.
5. A type of fission in

Answer:
1. Fertilisation
2. Testes
3. IVF
4. Oviparous
5. Binary
6. Internal
7. Buds
8. Ovary

NCERT Extended Learning Activities and Projects

Question 1.
Visit a poultry farm. Talk to the manager of the farm and try to find out the answers to the following.
a. What are layers and broilers in a poultry farm?
b. Do hens lay unfertilised eggs?
c. How can you obtain fertilised and unfertilised eggs?
d. Are the eggs that we get in the stores fertilised or unfertilised?
e. Can you consume fertilised eggs?
f. Is there any difference in the nutritional value of fertilised and unfertilised eggs?
Hint:
a. Layers are chickens raised for eggs and broilers are chickens raised for meat.
b. Yes, hens lay both the fertilised egg (if mating has occurred) and the unfertilised egg (according to light patterns if no mating has occurred).
c. Fertilised eggs are obtained by allowing hens to mate with roosters. Unfertilised eggs are obtained by exposing hens to suitable light patterns.
d. They are mostly unfertilised eggs.
e. Yes, fertilised eggs can be consumed.
f. No, there is no difference in the nutritional value of fertilised and unfertilised egg.

Question 2.
Observe live Hydra yourself and learn how they reproduce by doing the following activity:
During the summer months, collect water weeds from ponds or ditches along with the pond water and put them in a glass jar. After a day or so you may see several hydra clinging to the sides of the jar.
Hydra is transparent, jelly-like and with tentacles. It clings to the jar with the base of its body. If the jar is shaken, the hydra will contract instantly into a small blob, at the same time drawing its tentacles in. Now take out few hydras from the jar and put them on a watch glass. Using a hand lens or a binocular or dissection microscope, observe the changes that are taking place in their body. Note down your observations.
Hint:
Do it yourself.

Question 3.
The eggs we get from the market are generally the unfertilised ones. In case you wish to observe a developing chick embryo, get a fertilised egg from the poultry or hatchery which has been incubated for 36 hours or more. You may then be able to see a white disc-like structure on the yolk. This is the developing embryo. Sometimes if the heart and blood vessels have developed you may even see a red spot.
Hint:
Do it yourself.

Question 4.
Talk to a doctor. Find out how twinning occurs. Look for any twins in your neighbourhood, or among your friends. Find out if the twins are identical or non-identical. Also find out why identical twins are always of the same sex? If you know of any story about twins, write it in your own words.
Hint:
Twins are identical when they are formed due to a random splitting in the developing embryo which then develops into two different individuals. Since they are formed from the same zygote, their sex is always the same. However, non-identical twins are formed by the fertilisation of two different egg cells.

Activity 1

Objective: To observe and study the eggs of frog.
Procedure: Visit some ponds or slow-flowing streams during spring or rainy season and observe the colour and size of frog’s eggs floating on the water.
Observation: Amphibian eggs do not have a hard protective shell like the eggs of birds or reptiles. Frogs and toads lay eggs in jelly-like masses, and unless the eggs are covered with water, they will dry up and die.

Most amphibian eggs become much bigger after they are laid because the eggs absorb water and swell up to several times their original volume. The colour of the eggs is dull white and size ranges from less than a centimetre to a few centimetres.

Asexual Reproduction: The type of reproduction in which only a single parent is involved is called asexual reproduction. It can be of the following major types:
i. Budding: This method is seen in multicellular organisms which are highly simple in structure. A small bud develops on the body. The bud grows and starts resembling its parent cell. After sufficient growth, the bud gets detached from the parent’s body to begin life as a new individual. Examples: Hydra and sponges reproduce by budding.

Activity 2

Objective: To study budding in Hydra with the help of permanent slide.
Materials Required: Permanent slide of Hydra and a compound microscope.
Procedure:

• Get permanent slides of Hydra, observe them using a hand lens or a microscope.
• First observe the slide under low power and then under high power.
• Note the features and draw rough sketches.

Observations:

• Hydra uses regenerative cells for reproduction in the process of budding.
• In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site.
• These buds develop into tiny individuals and when fully mature, they detach from the parent body and become new independent individuals.

Conclusion: Budding is an asexual mode of reproduction as it involves only a single parent.

ii. Binary Fission: This method is seen in unicellular organisms, e.g., Amoeba. Binary fission begins with nuclear division where the parent nucleus divides into two nuclei. This is followed by cytoplasmic division, with each part getting one nucleus. In this way, the cell of Amoeba divides into two daughter cells. Each daughter cell becomes a new individual.

Cloning: The offspring which is morphologically as well as genetically identical to its parent is called a clone. The process of producing clones is called cloning. In the process of cloning of Dolly, a cell was collected from the mammary gland of female Finn Dorsett sheep. Simultaneously an egg was obtained from a Scottish blackface ewe. The nucleus was removed from the egg cell and mammary gland cell’s nucleus was inserted in it. Development of egg followed normally and finally Dolly was born on 5lh July, 1996 in Scotland and was the first mammal to be cloned. It was found to be absolutely identical to the Finn Dorsett sheep, from which the nucleus was taken.

Class 8 Science Chapter 9 Reproduction in Animals Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is sexual reproduction?
Answer:
When two parents are involved in the process of reproduction and gamete formation takes place, it is called sexual reproduction.

Question 2.
What is asexual reproduction?
Answer:
When a single parent is involved in reproduction and gamete formation does not take place, it is called asexual reproduction.

Question 3.
What is conception?
Answer:
Implantation of embryo in the wall of the uterus is called conception.

Question 4.
Give some examples of oviparous animals.
Answer:
Hen, lizards, all birds, etc.

Question 5.
Give some examples of viviparous animals.
Answer:
Cow, dog, cat and human beings.

Question 6.
Give the stages of development of a silk worm.
Answer:
Egg → Larva → Pupa → Adult

Question 7.
Name the male and female gametes.
Answer:
Mate gamete: Sperm Female gamete: Ovum

Question 8.
Name the reproductive organs of male and female.
Answer:
Male reproductive organs: Testes sperm ducts, penis.
Female reproductive organs: Ovaries, oviduct, uterus.

Question 9.
Name the male sex hormone.
Answer:
Testosterone.

Question 10.
Give the other name of oviduct.
Answer:
Fallopian tube.

Question 11.
What do you understand by the term ‘gestation’?
Answer:
It is the period between fertilisation and child birth, in which a foetus develops.

Question 12.
Name the female sex hormone.
Answer:
Estrogen.

Question 13.
Write the full form of IVF.
Answer:
In-vitro fertilisation.

Question 14.
Where does the development of embryo take place in the body of a female?
Answer:
The development of embryo takes place in the uterus of the female body.

Question 15.
What is foetus?
Answer:
A well developed embryo with distinct and identifiable body parts is called foetus.

Question 16.
Name the various methods of asexual reproduction in animals.
Answer:
Budding, fission, regeneration and fragmentation.

Question 17.
What type of reproduction takes place in Amoeba?
Answer:
Asexual reproduction (binary fission).

Short Answer Type Questions

Question 1.
What is the difference between sexual and asexual reproduction?
Answer:

Question 2.
Briefly explain in-vitro fertilisation.
Answer:
Any biological process carried out in laboratory is called ‘in-vitro’ process. Thus, fertilisation carried out in laboratory is called in-vitro fertilisation. Many women may fail to conceive due to certain problems. In-vitro fertilisation helps such women to bear a child. In this technique, a freshly released sperm (from the father) and an ovum (from the mother) are collected and allowed to fuse under artificial conditions. After in-vitro fertilisation, zygote is allowed to develop into a mass of cells and is then transferred into the uterus for subsequent development.

Question 3.
Explain the steps of development of embryo.
Answer:
Conception and child birth: It takes place in the female reproductive system through the following steps:

• A single egg comes out of the ovary every month and reaches the fallopian tube.
• Sperms reach the fallopian tube where a sperm fertilises the egg. This results in the formation of zygote.
• Zygote undergoes several rounds of cell division to become a ball of cells.
• The cells begin to form groups that develop into different tissues and organs of the body. This structure is called embryo.
• This embryo moves down and gets implanted in the wall of the uterus. This process is called conception. Conception marks the beginning of pregnancy.
• Embryo develops further inside the uterus to form various body parts such as hands, head, eyes, etc. This stage of embryo is called foetus.
• When the development of the embryo is complete, the mother gives birth to the baby.

Question 4.
Describe the male reproductive organs.
Answer:
The male reproductive organs are a pair of testes, sperm ducts and a penis. Testes produce male gametes called sperms. The two sperm ducts open into the top of urethra. Urethra carries both the sperms and the urine. The urethra is a narrow tube and runs through the penis to the outside.

Question 5.
Differentiate between budding and binary fission.
Answer:

Question 6.
Describe the female reproductive organs.
Answer:
Female reproductive organs include a pair of ovaries, oviduct and the uterus. In human beings, a single matured egg is released into the oviduct by one of the ovaries every month. Fallopian tube carries the egg from ovaries to the uterus. Uterus is the part where development of the baby takes place.

Question 7.
Define sperm. Describe its structure.
Answer:
The testes secrete millions of sperm cells together. A sperm comprises a single cell and has a specific structure with three main parts as given below:
a. Head: It consists of the nucleus which contains the DNA information of the cell.
b. Middle Piece: It is packed with cell organelle called mitochondria. The mitochondria are responsible for producing energy in the cell. Sperm uses this energy to move.
c. Tail: It allows the sperm cell to travel at a fast pace.

Question 8.
Define ovum. Describe its structure.
Answer:
Ovum or egg is the female gamete which produces zygote after fertilisation with the male gamete. An ovum is single-celled and spherical in shape. If consists of a prominent nucleus and a dense cytoplasm which is full of nutrition.

Question 9.
What are test-tube babies?
Answer:
In some women, the oviducts are blocked and hence the eggs are unable to reach the uterus. This means that these women are not able to bear babies because of blocked oviducts. However, due to the process of in-vitro fertilisation or IVF, the freshly released eggs of females and the male sperms can be fertilised externally. The zygote thus formed is allowed to develop for a week outside the female body in safe conditions and is then placed in the mother’s uterus. In this way, these women can bear babies. The babies that are born with the IVF process are called test-tube babies.

Question 10.
Why are gametes produced in large numbers in organisms that exhibit external fertilisation?
Answer:
Organisms that take part in external fertilisation produce a large number of gametes because:
a. in external fertilisation, there is a great chance that the sperms and the eggs released by the organisms can be affected by the environmental factors, like desiccation, predators, etc. So, to compensate for the high mortality rate of the gametes, the organism produces a large number of gametes.

b. producing a large number of gametes increases the chances for at least some eggs and sperms to fuse in the environment ensuring that at least a stable number of offspring are produced.

Question 11.
How are chicks born? Explain their developmental process.
Answer:
After fertilisation the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell on the hen’s egg is one such protective layer. The hen finally lays the egg. The embryo takes about three weeks to develop into a chick. After the chick is completely developed, it bursts open the egg shell and hatches out.

Question 12.
How are general growth and sexual maturation different from each other?
Answer:
General growth refers to different types of developmental process in the body like increase in height, weight gain, changes in shape and size of the body but sexual maturation is specific to change reflected at puberty like cracking of voice, new hair patterns, development of breast in female, etc.

Question 13.
Explain the life cycle of silkworm in brief.
Answer:
The adult lays eggs on mulberry leaves. The caterpillar hatches out from the egg and feeds actively. If then converts itself into pupa. The pupa stage is inactive. This pupa finally metamorphosis into the adult silk moth. Hence, following stages can be summed up in the life cycle of a silkmoth:
Egg → Larva or Caterpillar → Pupa → Adult

Question 14.
Explain the process of fertilisation in frogs.
Answer:
Frogs undergo external fertilisation. During rainy season, they move to ponds and slow flowing streams. The female lays hundreds of eggs which are not covered by a shell and are very delicate. A layer of jelly holds the eggs together and provides protection. The male deposits sperms over these eggs. The sperms swim randomly in water and fuse with the eggs resulting in fertilisation.

Question 15.
Birth of a new born baby in a family is rejoicing and celebrating. Unfortunately, about 3% of the new born suffer from one of the other birth defects.
a. What are the factors that cause such defects?
b. List the different techniques being used towards parental diagnosis of such anomalies.
c. How are these techniques useful in counselling for such cases?
Answer:
a. Anomalies are due to a number of genetic and environmental factors.
b. A number of techniques are available nowadays for detecting such disorders. These include blood test of the mother and ultrasound, etc.
c. Based on these reports, expert advice is sought for continuation of pregnancy through suitable medical treatment or for termination of pregnancy.

Long Answer Type Questions

Question 1.
What is reproduction? Describe the various modes of reproduction.
Answer:
Reproduction is the process by which living organisms produce more living organisms of their own kind. It helps in the continuation of a species on the earth.

Modes of Reproduction: There are two main modes of reproduction in living organisms:
a. Asexual reproduction: The mode of reproduction in which a single organism is able to produce one or more organisms of its kind by itself is called asexual reproduction. Asexual reproduction does not require the involvement of both the male and the female. The organisms like Amoeba, yeast, Hydra, etc., reproduce by this mode of reproduction.

b. Sexual reproduction: In sexual mode of reproduction, both the parents (male and female) are needed to produce individuals of the same kind. Fish, frog, higher animals such as cow, dog, horse and human beings reproduce by the mode of sexual reproduction.

Question 2.
Describe the formation of egg shell in hens.
Answer:
Internal fertilisation takes place in hens. After fertilisation, the zygote divides continuously and moves to the oviduct. As it travels down, many protective layers are formed around it. The hard shell in a hen’s egg is one such protective layer. After the hard shell is formed, the hen finally lays the egg. The embryo takes about 3 weeks to develop into a chick.

The hen sits on the eggs to provide sufficient warmth. After the chick is completely developed, it bursts open the egg shell.

Question 3.
Define cloning. Describe the formation of Dolly.
Answer:
Cloning is the process of production of an exact copy of a cell, any other living part, or a complete organism. Cloning process of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly.

During the process of cloning a sheep named Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was collected from a Scottish blackface ewe. The nucleus was removed from the egg. After that, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. Then this egg was implanted into the Scottish blackface ewe. Development of this egg followed normally and then finally Dolly was born. It was found to be absolutely identical to the Finn Dorsett sheep, from which the nucleus was taken.

Picture-Based Questions

Question 1.
Draw a labelled diagram to show the male reproductive system in human beings.
Answer:

Question 2.
Draw a labelled diagram to show the female reproductive system in human beings.
Answer:

Question 3.
Observe the given figure and answer the questions that follow.

a. Name the components labelled A and B.
b. Identify the process.
c. What happens during the process and what is formed?
Answer:
a. A-sperms, B-ovum (egg).
b. The process is fertilisation.
c. During the process, the sperm nucleus fuses with the egg nucleus; as a result of which, a zygote is formed.

Question 4.
Identify the following diagram and answer the questions:

a. Where does the embryo develop in human body?
b. How many days does an embryo take to develop into a fully mature foetus.
Answer:
The diagram shows the development of foetus inside the uterus.
a. The embryo develops inside the uterus of the human female body.
b. The embryo takes about 270 to 280 days to develop into a fully mature foetus.

Question 5.
a. Draw the diagram of a human sperm and label its head, middle piece and tail.
b. What is the function of its tail?
Answer:
a.

b. The tail of the sperm helps it in swimming in the liquid medium.