# Question: 1

A list of mediums is given below:

(i) Wood

(ii) Water

(iii) Air

(iv) Vacuum

In which of these mediums can sound travel?

(a) i& ii only

(b) i, ii & iii only

(c) iii & iv only

(d) ii, iii & iv only

b

# Question: 2

The loudness of sound depends on:

(a) Its amplitude.

(b) Its frequency.

(c) Its time period.

(d) Its speed.

a

# Question: 3

Which of the following statements are correct?

(i) Sound is produced by vibrations.

(ii) Sound requires a medium for propagation.

(iii) Light and sound both require a medium for propagation.

(iv) Sound travels slower than light.

(a) i & ii only

(b) i, ii & iii only

(c) ii, iii & iv only

(d) i, ii & iv only

d

# Question: 4

An object is vibrating at 50 hertz.

What is its time period?

(a) 0.02 s

(b) 2 s

(c) 0.2 s

(d) 20.0 s

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.

(c) Decrease its amplitude of vibration of the sound.

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

c

# Question: 6

Loudness of sound is measured in units of:

(a) Decibel (dB)

(b) Hertz (Hz)

(c) Metre (m)

(d) Metre/second (m/s)

a

# Question: 7

The loudness of sound is determined by the:

(a) Amplitude of vibration

(b) Ratio of amplitude and frequency of vibration

(c) Frequency of vibration

(d) Product of amplitude and frequency of vibration

a

# Question: 8

1 hertz is equal to:

(a) 1 vibration per minute

(b) 10 vibrations per minute

(c) 60 vibrations per minute

(d) 600 vibrations per minute

c

# Question: 9

Pitch of sound is determined by its:

(a) Frequency

(b) Amplitude

(c) Speed

(d) Loudness

a

# Question: 10

Ultrasound has frequency of vibration:

(a) Between 20 and 20,000 Hz

(b) Below 20 Hz

(c) Above 20,000 Hz

(d) Between 500 and 10,000 Hz

c

# 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).

## Solution:

So, Paheli is 1650 m away from the place where lightning occurred.

# Question: 12

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

## Solution:

The part of our body that vibrates when we speak is the 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.

## Solution:

The speed of light is greater than the speed of sound, therefore sound reaches Boojho later than light.

# Question: 14

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

## Solution:

The part of our body that vibrates when we hear sound is the eardrum.

# Question: 15

Name two musical instruments which produce sound by vibrating strings?

## Solution:

Sitar and guitar are the two musical instruments which produce sound by vibrating strings.

# Question: 16

A simple pendulum makes 10 oscillations in 20 seconds.

What is the time period and frequency of its oscillation?

## Solution:

Frequency

= Number of oscillations/Time

Time Period

$=\frac{1}{Frequency}$

# 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?

## Solution:

We can hear the sound produced by any vibrating body, if the sound produced is within the audible range. We will not be able to hear any other sound falling outside this range, 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.

## Solution:

The frying pan will vibrate. Since sound cannot travel through vacuum, we will not be able to hear the sound of vibration.

# 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.

## Solution:

As we know that sound cannot travel through vacuum, so, astronauts will not be able to talk to each other.

# Question: 20

List three sources of noise pollution in your locality.

## Solution:

Loud horns, fireworks and loudspeakers

# 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?

## Solution:

The amplitude of vibration determines the loudness of sound. When the string is plucked in the middle with a greater force of magnitude, the amplitude would be larger. Therefore, the sound produced will be louder.

# Question: 22

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

## Solution:

As we see water waves travelling along the surface of a water body in ripples, sound waves also travel in the same manner through the air, moving particles coming along their paths. These moving particles and their resulting air pressure have an impact on human eardrums and transmit the vibrations to them. When these vibrations finally reach our brain, we get the sensation that we have heard a sound or a noise.

# Question: 23

An alarm bell is kept inside a vessel as shown in the following figure. 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?

## Solution:

As the air is removed slowly from the plastic bottle, the loudness of the sound too decreases. Vacuum will be created in the bottle if the air is completely removed. As sound cannot travel through vacuum, we will not be able to hear the sound of the alarm clock at all.

# Question: 24

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

## Solution:

The sound of the clock appears much clearer at night than in the day because the interfering noise level is lower at night.

# Question: 25

Suggest three measures to limit noise pollution in your locality.

## Solution:

(i) Plant more trees along the roads and around the buildings.

(ii) Use of loud horns must be minimised or prohibited by law.

(iii) Transportation vehicles and industrial machineries must have silencers installed, to minimise sound pollution.