Chapter 13: Sound

Science

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Chapter 13: Sound

Class: VIII

Exemplar Solutions

MULTIPLE CHOICE QUESTIONS

Question 1

A list of mediums is given below.

(i)wood (iii) air

(ii) water (iv) vacuum

In which of these mediums can sound travel?

(a) i & ii only (c) iii & iv only

(b) i, ii & iii only (d) ii, iii & iv only

Answer 1 (b)

Question 2

The loudness of sound depends on:

(a) its amplitude. (c) its time period.

(b) its frequency. (d) its speed.

Answer 2 (a)

Question 3

Which of the following statements are correct?

Sound is produced by vibrations.
Sound requires a medium for propagation.
Light and sound both require a medium for propagation.
Sound travels slower than light.

(a) i & ii only

(b) i, ii & iii only

(d) i, ii & iv only

Answer 3 (d)

Question 4

An object is vibrating at 50 hertz. What is its time period?

(a) 0.02s (c) 0.2 s

(b) 2s (d) 20.0s

Answer 4 (a)

Question 5

In order to reduce the loudness of a sound we have to

(a) decrease its frequency of vibration of the sound.

(b) increase its frequency of vibration of the sound.

(d) increase its amplitude of vibration of the sound.

Answer 5 (c)

Question 6

Loudness of sound is measured in units of

(a) decibel (dB) (c) metre (m)

(b) hertz (Hz) (d) metre/second (m/s)

Answer 6 (a)

Question 7

The loudness of sound is determined by the

(a) amplitude of vibration

(b) ratio of amplitude and frequency of vibration

(d) product of amplitude and frequency of vibration

Answer 7 (a)

Question 8

1 hertz is equal to

(a) 1 vibration per minute

(b) 10 vibrations per minute

(d) 600 vibrations per minute

Answer 8 (c)

Question 9

Pitch of sound is determined by its

(a) frequency (c) speed

(b) amplitude (d) loudness

Answer 9 (a)

Question 10

Ultrasound has frequency of vibration

(a) between 20 and 20,000 Hz

(b) below 20 Hz

(d) between 500 and 10,000 Hz

Answer 10 (c)

VERY SHORT ANSWER QUESTIONS

Question 11

Lightning can be seen the moment it occurs. Paheli observes lightning in her area. She hears the sound 5 s after she observed lightning. How far is she from the place where lightning occurs? (speed of sound = 330 m/s).

Answer 11

1650 m

Question 12

Does any part of our body vibrate when we speak? Name the part.

Answer 12

Yes, larynx (vocal cords)

Question 13

Boojho saw a cracker burst at night at a distance from his house. He heard the sound of the cracker a little later after seeing the cracker burst. Give reason for the delay in hearing the sound.

Answer 13

The speed of sound is lower than that of the speed of light and therefore sound reaches him later than light.

Question 14

When we hear a sound, does any part of our body vibrate? Name the part.

Answer 14

Yes, eardrum.

Question 15

Name two musical instruments which produce sound by vibrating strings?

Answer 15

Sitar and Ektara (any other musical instruments which produces sound by a vibrating string)

Question 16

A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?

Answer 16

Time period : 2 s

frequency : 0.5 oscillations/sec

Question 17

We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?

Answer 17

If the sound produced by a vibrating body is in the audible range, the sound produced will be heard by us otherwise we will not be able to hear the sound even though the body is vibrating.

Question 18

Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.

Answer 18

The frying pan will vibrate. We will not be able to hear the sound of vibration because sound cannot travel in vacuum.

Question 19

Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.

Answer 19

No, In space there is vacuum and sound cannot travel in vacuum.

Question 20

List three sources of noise pollution in your locality.

Answer 20

Vehicle noise, bursting of crackers, loudspeakers.

(Any other reasonable sources of noise pollution should be accepted)

LONG ANSWER QUESTIONS

Question 21

We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude and then with a force of smaller magnitude. In which case would the instrument produce a louder sound?

Answer 21

The loudness of sound depends upon the amplitude of vibration. The amplitude of string is larger when it is plucked with greater force and hence the sound will be louder in that case.

Question 22

How is sound produced and how is it transmitted and heard by us?

Answer 22

Hint: Explain how a vibrating body produces sound and how it travels through the air and is heard by us by our ears.

Question 23

An alarm bell is kept inside a vessel as shown in Fig. 13.1. A person standing close to it can distinctly hear the sound of alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?

Answer 23

The loudness of the sound will decrease as the air is removed slowly from the plastic bottle. If the air in the plastic bottle is removed completely, there is vacuum in the bottle. The sound cannot travel through vacuum and we cannot hear the sound of the alarm clock at all.

Question 24

The townhall building is situated close to Boojho’s house. There is a clock on the top of the townhall building which rings the bell every hour. Boojho has noticed that the sound of the clock appears to be much clearer at night. Explain.

Answer 24

The noise level is quite low at night. Therefore the sound of the clock appears much clearer at night than in the day.

Question 25

Suggest three measures to limit noise pollution in your locality.

Answer 25