NCERT Class 8 Science Exemplar Chapter 13 - Sound

Chapter 13 - Sound

Multiple Choice Questions
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
Answer:
b) i, ii & iii only

Question 2: The loudness of sound depends on:
a) its amplitude.
b) its frequency.
c) its time period.
d) its speed.
Answer:
a) its amplitude.

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
Answer:
d) i, ii & iv only

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
Answer:
a) 0.02 s

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.
Answer:
c) decrease its amplitude of vibration of the sound.

Question 6: Loudness of sound is measured in units of
a) decibel (dB)
b) hertz (Hz)
c) metre (m)
d) metre/second (m/s)
Answer:
a) decibel (dB)

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
Answer:
a) amplitude of vibration

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
Answer:
c) 60 vibrations per minutes

Question 9: Pitch of sound is determined by its
a) frequency
b) amplitude
c) speed
d) loudness
Answer:
a) frequency

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
Answer:
c) above 20,000 Hz

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:

Light always travels faster than sound, so we see the lighting first and then the sound. We can detect the how far the lightning had happened from us.
Distance = Time × Speed
Let distance be x.
Time = 5 seconds
Speed = 330 m/s
= 330 × 5 = 1650 m

Therefore, the distance between Paheli and the lightning is 1650 m.

Question 12: Does any part of our body vibrate when we speak? Name the part.
Answer:
Yes, the body part that vibrates when we speak is larynx or the 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:
The delay in hearing of sound is because the speed of light is much faster than the speed of sound. Therefore, Boojho heard the sound of the cracker a little later after seeing the cracker burst.

Question 14: When we hear a sound, does any part of our body vibrate? Name the part.
Answer:
Yes, the body part that vibrates when we hear a sound is ear drum.

Question 15: Name two musical instruments which produce sound by vibrating strings?
Answer:
The two musical instruments which produce sound by vibrating strings are sitar and ektara.

Short Answer Questions
Question 16: A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?
Answer:

Number of oscillations = 10
Time taken = 20 seconds
Frequency = Number of oscillations/Time taken
                    = 10/20 = 0.5 Hz
Time period = 1/Frequency
                      = 1/0.5
                      = 2 seconds

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:
Sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz. Hence, 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:
Yes, the frying pan will vibrate but we will not be able to hear the sound of vibration because sound cannot travel in vacuum. Sound can’t travel in vacuum because the sound needs mediums to travel.

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:
No, the two astronauts can’t talk to each other without using any special device, because in the space there is vacuum which doesn’t allow sound to pass through it.

Question 20: List three sources of noise pollution in your locality.
Answer:
The three sources of noise pollution in our locality
• Car horns
• Bursting crackers
• Loud music playing during the party or festival.

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:
The string that is plucked in the middle first with a force of greater magnitude will have louder sound because the loudness of sound depends upon the amplitude of vibration.

Question 22: How is sound produced and how is it transmitted and heard by us?
Answer:
Sound is a form of energy that enables us to hear and it travels in longitudinal waves. When matter vibrates or moves back and forth very quickly a sound is made. Sound waves travels through solids, liquids and gases. Sound can’t travel in vacuum because no particles are present. Sound travels fastest in solids because of the molecules are closer while the sound travels slowest in gases because of the molecules are far.
Sound waves are sent. The outer ear catches the sound waves. The middle ear takes the sound waves and vibrate the eardrum. The inner ear sends the messages to the brain. The brain puts in together and we can understand sound.

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?

Fig 13.1

Answer: 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 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.
Answer:
When sound is produced, speed, pitch, loudness all are initiated with vibration. During the day, there are many vibrations around us. This results in a decrease of the amplitude of vibrations. During the night, there will be no such disturbances which will make the sound clearer. And therefore, Boojho noticed the sound of the clock clear during night.

Question 25: Suggest three measures to limit noise pollution in your locality.
Answer:
The measures to limit noise pollution in our locality are
→ Trees must be planted along the roads and around the buildings because trees help to reduce the loudness of sound.
→ Use of horns should be minimised.
→ We shouldn’t burst crackers.

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