NCERT Solution Sound Chap 10 Exercises with answer

 NCERT Solution

Sound Chap 10

Exercises

Question 1

Sound can travel through

1.     gases only

2.     solids only

3.     liquids only

4.     solids, liquids and gases

Answer

solids, liquids and gases

Reason — Sounds needs a medium to travel. Solids, liquids and gases provide a medium through which sound can travel.

Question 2

Voice of which of the following is likely to have a minimum frequency?

1.     Baby girl

2.     Baby boy

3.     A man

4.     A woman

Answer

A man

Reason — The frequency of a sound is proportional to its pitch. An adult man's voice has a lower pitch than the voices of a baby boy, a baby girl, and a woman.

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 a 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 the vibration is large, the 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) Sound cannot travel in a vacuum. — True

(b) The number of oscillations per second of a vibrating object is called its time period. — False
Reason — The number of oscillations per second of a vibrating object is called its frequency.

(c) If the amplitude of the vibration is large, the sound is feeble. — False
Reason — If the amplitude of the vibration is large, the sound is loud.

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. — True

(e) The lower the frequency of vibration, the higher is the pitch. — False
Reason — The lower the frequency of vibration, the lower is the pitch.

(f) Unwanted or unpleasant sound is termed as music. — False
Reason — Unwanted or unpleasant sound is termed as noise.

(g) Noise pollution may cause partial hearing impairment. — 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 taken by an object to complete one oscillation is called time period.

(b) Loudness is determined by the amplitude of vibration.

(c) The unit of frequency is hertz.

(d) Unwanted sound is called noise.

(e) The shrillness of a sound is determined by the frequency of vibration.

Question 5

A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer

Given,

Total number of oscillations = 40

Total time taken = 4 s

We know,

Frequency = Total number of oscillationsTotal / time 

Substituting we get,

Frequency = 10​ = 10 Hz

Time period = 1 / Frequency​

Substituting we get,

Time period = 1 / 10​ = 0.1 s

Hence, frequency = 10 Hz and time period = 0.1 s.

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

Given,

Total number of vibrations = 500

Total time taken = 1 s

We know,

Time period = 1 / Frequency​ and

Frequency = Total number of oscillations / Total time

Hence,

Time period = Total time taken / Total number of oscillations  ​

Substituting we get,

Time period = 1 / 500​ = 0.002 s

So, time period = 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) Dholak — Stretched membrane.

(b) Sitar — Stretched strings

(c) Flute — Air column

Question 8

What is the difference between noise and music? Can music become noise sometimes?

Answer

Noise	Music
Noises are sounds which are unpleasant to hear.	Musical sound is one which is pleasing to the ear.
Example: Horns of buses and trucks.	Example: Sounds from flutes, pianos and violins.

Yes, when music is played at very high volume, it becomes noise.

Question 9

List sources of noise pollution in your surroundings.

Answer

Sources of noise pollution are:

1.     Horn of vehicles

2.     Explosions including bursting of crackers

3.     Sounds from machines, loudspeakers

4.     Television, transistor and radio at high volumes

5.     Desert coolers, air conditioners

Question 10

Explain in what way noise pollution is harmful to humans.

Answer

Presence of excessive noise in the surrounding may cause many health related problems like:

1.     Lack of sleep

2.     Hypertension

3.     High blood pressure

4.     Anxiety

5.     Temporary or permanent impairment 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'd suggest that they go for the house that's three lanes away from the roadside. Being further from the roadside reduces the level of noise pollution from passing vehicles, making it a quieter and more peaceful living environment.

Question 12

Sketch the larynx and explain its function in your own words.

Answer

The diagram of larynx is shown below:

Larynx is known as the voice box in the human body. There are two vocal cords across the larynx. The air passes through a small gap which is present in between them. The lungs force the air into the gap when we speak, and this vibrates the vocal cord, due to which sound is produced.

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

Thunder is heard a few seconds after lightning is seen because sound travels much slower than light.

Light takes almost negligible time in comparison to sound in reaching us from the place of thunder because speed of light is much more (3 x 10⁸ ms^-1) than the speed of sound (= 330 ms^-1).

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NCERT Solution Friction Chap 9 for Class 8

 

NCERT Solution

Friction Chap 9

Exercises

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) Friction opposes the motion between the surfaces in contact with each other.

(b) Friction depends on the nature of surfaces.

(c) Friction produces heat.

(d) Sprinkling of powder on the carrom board reduces friction.

(e) Sliding friction is smaller than the static friction.

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.

1.     rolling, static, sliding

2.     rolling, sliding, static

3.     static, sliding, rolling

4.     sliding, static, rolling

Answer

static, sliding, rolling

Reason —

• Static friction is usually the highest among the three types because it's the force required to overcome the initial inertia and get an object moving from rest.
• Once the object is in motion, the contact points on the object do not get enough time to lock into the contact points of the other surface. So, the sliding friction is slightly smaller than the static friction.
• Rolling friction is least as rolling motion reduces the contact area between the object and the surface, thus reducing the friction. Hence, the decreasing order of friction is 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 :

1.     wet marble floor, dry marble floor, newspaper and towel

2.     newspaper, towel, dry marble floor, wet marble floor

3.     towel, newspaper, dry marble floor, wet marble floor

4.     wet marble floor, dry marble floor, towel, newspaper

Answer

wet marble floor, dry marble floor, newspaper and towel.

Reason — The force of friction depends on several factors, including the roughness of the surfaces and presence of moisture.

• Movement on towel offers high friction because its surface is usually rough, providing significant resistance to motion.
• Newspaper is smoother than a towel but still provides some resistance due to its texture.
• Dry marble is smoother than newspaper, so it has less friction compared to the previous two surfaces.
• Wet marble has the least friction among the listed surfaces because water acts as a lubricant, reducing the friction between the car's tires and the surface of the wet marble.

Hence, the order of increasing friction will be: wet marble floor < dry marble floor < newspaper < towel.

Question 4

Suppose you’re 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 the book slides down on the desk, the frictional force acts opposite to the direction of motion of book (i.e., it acts in the upward direction).

Question 5

You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer

Spilling soapy water on a marble floor would make it more difficult to walk on the floor. When we push the floor in the backward direction with our feet, the force of friction pushes it in the forward direction and allows us to walk. The soap in the water creates a slippery surface, reducing friction between our feet and the marble floor. As a result, walking becomes more difficult on the slippery marble floor.

Question 6

Explain why sportsmen use shoes with spikes.

Answer

Sportsmen use shoes with spikes to run faster, improve their stability and avoid injury. By digging into the ground, the spikes increase the surface area contacting the playing surface, leading to greater friction. This increased friction enables faster running and reduces the risk of slipping.

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

Seema has to apply greater force than Iqbal because her box is heavier and heavier object will have greater frictional force with the surface of contact. This happens because friction is caused by the interlocking of irregularities in the two surfaces. The force of friction will increase if the two surfaces are pressed harder. Hence, more force is required to push the heavier box.

Question 8

Explain why sliding friction is less than static friction.

Answer

Static friction is the force required to overcome the initial inertia and get an object moving from rest. Sliding friction comes into action when the object is in motion. Here, the contact points on the object do not get enough time to lock into the contact points of the other surface. So, the sliding friction is smaller than the static friction.

Question 9

Give examples to show that friction is both a friend and a foe.

Answer

Friction as a friend:

1.     We are able to walk on floor due to friction between floor and our feet.

2.     We are able to write due to friction between pen and paper.

3.     An object in motion will never stop if there is no friction.

4.     Without friction between tyres of automobiles and roads, they could not be started, stopped or turned to change direction of motion.

5.     Friction helps us to tie a knot.

Friction as a foe:

1.     It wears out the materials like screws, ball bearing or soles of shoes.

2.     Friction produces heat and so the jar of mixer becomes hot when it runs for a few minutes.

3.     Friction slows down moving objects, hence, more force is required.

4.     Due to friction blades of scissors or knives become blunt.

5.     Due to increased friction between tire and the road surface, when the tire deflates, it becomes difficult to move the vehicle.

Question 10

Explain why objects moving in fluids must have special shapes.

Answer

Objects moving through fluids experience drag, a frictional force exerted by the fluids. Special shapes, such as streamlined designs, are essential to minimize drag. These shapes reduce disturbance and redirect fluid flow around the object, improving efficiency and helping the object overcome resistance as it moves through the fluid.

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NCERT Solution Force and Pressure Chap 8 for Class 8

 

NCERT Solution

Force and Pressure Chap 8

Exercises

Question 1

Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer

(a) Pull situations (in which state of motion of objects is changed):

1.     Moving a book placed on a table towards ourself to read.

2.     We pull the clothes out of the washing machine after they are clean.

(b) Push situations (in which state of motion of objects is changed):

1.     A football player taking a penalty kick, hits (or pushes) the ball towards the goal.

2.     A cricket ball hit by a batsman.

Question 2

Give two examples of situations in which applied force causes a change in the shape of an object.

Answer

1.     When we press a lump of dough kept on a plate with our hands, its shape changes.

2.     When we press a smiley ball, its shape changes.

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) To draw water from a well, we have to pull at the rope.

(b) A charged body attracts an uncharged body towards it.

(c) To move a loaded trolley, we have to pull or push it.

(d) The north pole of a magnet repels the north pole of another magnet.

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) To stretch the bow, the archer applies a force that causes a change in its shape.

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

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

S. No.	Agent	Object	Effect	Reason
a.	Fingers	Lemon	Shape of lemon changes	Muscular force is exerted on the lemon in order to extract its juice. As a result, the shape of the lemon changes.
b.	Fingers	Tooth paste tube	Shape of tooth paste tube changes	Muscular force is exerted on the tube in order to take out paste. As a result, the shape of the tube changes.
c.	Suspended load	Spring	The spring gets stretched.	The suspended load pushes the spring downwards. Hence, its shape changes.
d.	Athlete	Ground	Pushes the ground backwards and as a result takes a high jump to clear the bar at a certain height.	When the athlete pushes the ground with his feet, his feet exert a muscular force on the ground. Same magnitude of force is applied back by the ground on the athlete. This force allows them to jump over the bar. As a result, their 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

The muscular force applied by the blacksmith on the hot piece of iron changes the shape of the iron and hence, it can be moulded in any desired shape.

Question 7

An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon stuck to the wall. What force might be responsible for the attraction between the balloon and the wall?

Answer

An electrostatic charge develops on the balloon after it has been rubbed with a synthetic cloth. Due to this charge the balloon sticks to the wall. The electrostatic force is responsible for the attraction between the balloon and the wall.

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

We use muscular force in order to hold the bucket of water above the ground. This muscular force acts against the force of gravity that pulls the bucket towards the ground. The two forces are equal in magnitude and opposite in direction. As a result, the net force on the bucket is zero. Hence, there is no change in the state of motion.

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

When a rocket has been fired upwards to launch a satellite in its orbit, two forces acting on it are:

1.     Force of gravity acting downwards

2.     Frictional force due to the earth's atmosphere, which opposes its motion.

Question 10

When we press the bulb of a dropper with its nozzle kept in water, the 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

1.     pressure of water

2.     gravity of the earth

3.     shape of rubber bulb

4.     atmospheric pressure

Answer

atmospheric pressure

Reason — When all the air escapes from the nozzle, a low pressure develops inside the tube. Due to this the atmospheric pressure, acting on water becomes more than the pressure inside the tube. As water moves from a higher pressure area to a lower pressure area, hence on releasing the bulb, water gets filled in the dropper.

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