NCERT Exemplar Solution Class 8 Science Chapter 13
NCERT Exemplar Solution Class 8 Science Chapter 13 Sound all Questions Answer Solution. NCERT Exemplar Class 8 Science Chapter 13 Solution PDF.
NCERT Exemplar Solution Class 8 Science Chapter 13: Overview
NCERT Exemplar Solution Class 8 Science Chapter 13 |
|
Board |
NCERT |
Topic |
Exemplar Problem Solution |
Class |
8 |
Subject |
Science |
Chapter |
13 |
Chapter Name |
Sound |
NCERT Exemplar Solution Class 8 Science Chapter 13 Sound
Multiple Choice Questions
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 and II only
b) I, II, and III only
c) III and IV only
d) II, III, and IV only
Answer:- b) I, II, and III only
Sound needs an media for travel, wood, air, water becomes the medium for the travel of sound. But sound cannot travel through vacuum.
2) The loudness of sound depends on:
a) Its amplitude
b) It’s frequency
c) It’s time period
d) It’s speed
Answer:- a) It’s amplitude
Higher the Amplitude higher is the loudness of the sound. Thus the loudness is dependent on the amplitude.
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 and II only
b) I, II, and III only
c) II, III, and IV only
d) I, II and IV only
Answer:- d) I, II and IV only
All the statements are correct excluding the light and sound both require a medium for propagation, light does not required medium for travelling. But sound required medium to travel.
4) An object is vibrating at 50 hertz. What is its time period?
a) 0.02 s
b) 0.2 s
c) 2 s
d) 20.0 s
Answer:- a) 0.02 s
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 it’s amplitude of vibration of the sound.
The loudness of the sound depends upon the large force of the amplitude. To reduce the loudness we should decrease the amplitude of vibration of sound.
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)
The unit of measuring the loudness of sound is Decibel (dB). Frequency is measured in Hz. Distance of sound is measured in m/s. Length in metre.
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
Amplitude determine the loudness of vibration. Higher the Amplitude, louder is the sound.
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 minute.
1 oscillation per sec is the frequency of 1 Hz. Therefore, Per min i.e 60 sec required 60 vibrations.
9) Pitch of sound is determined by its
a) Frequency
b) Amplitude
c) Speed
d) Loudness
Answer:- a) Frequency
Lower the frequency lower is the pitch of sound, and higher the frequency higher is the pitch if sound. Pitch is dependent upon the frequency of sound.
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
Ultrasound is mostly used in the medical field, it’s frequency of vibration is higher than 20,000 Hz which is not audible by the human beings.
Very Short Answer Questions
11) Lightning can be seen the moment it occurs. Paheli observes lightning in her area. She hears the sound 5s after she observed lightning. How far is she from the place where lightning occurs?
(Speed of sound = 330 m/s)
Answer:- Speed of the light is higher than the speed of sound, therefore we see the light First and then we listen the sound.
Here given ,
Sound is listened after 5 s.
Speed of sound is 330 m/s.
Distance = speed × time
= 330 × 5
= 1650 m/s.
12) Does any part of our body vibrate when we speak? Name the part.
Answer:- yes, A part of our body vibrates , Named as Larynx or the voice box. These voice box is present at the upper end of the Windpipe, between two vocal cords across it. It has a small gap (slit) between the vocal cords for passage of air. The air forced out by lungsthrough the small gap(slit) and then the vocal cords vibrate to produce the sound.
13) Boojho saw a cracker burst at night at a distance 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 reason for the delaying in hearing the sound is that, the speed of light is higher than the speed of sound. Therefore we see light first and then after some seconds we listen the sound.
14) When we hear a sound, does any part of our body vibrate ? Name the part.
Answer:- yes, The part of our body vibrates when we hear sound is Eardrum. Eardrum is the thin and stretched membrane like an rubber sheet,A narrow canal reaches towards it. When any sound produced vibrations, the eardrum also vibrates. And these vibration is further sent to the inner ear, then the signal is send to the brain.
15) Name two musical instruments which produce sound by vibrating strings?
Answer:- The Names of instruments which produces sound by vibrations strings are –Guitar, Sitar and Ektara.
Short Answer Questions
16) A simple pendulum makes 10 oscillation in 20 seconds. What is the time period and frequency of its oscillation?
Answer:- Frequency is known as the number of oscillations per second. It is expressed in Hertz (Hz). Now, the one oscillation per second is the frequency of 1 Hz.
Therefore, now as per given information,
10 oscillations = 20 seconds
So, 1 oscillation = 2 seconds.
therefore frequency is 0.5 oscillations per sec.
Time period = 2 sec
Frequency = 0.5 oscillation / sec
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:-
We cannot hear sound produced by every vibration. Human beings cannot hear the sound which has the frequency less than 20 Hz. (20 vibrations per sec) . These sound is known as inaudible. And on the other side More than 20 kHz (20,000 oscillation per sec) is also not audible by the human beings. Human can only hear the sound only in-between the range of frequencies from 20 to 20000 Hz. This frequencies sound is audible to humans.
18) Suppose a stick is stuck against a flying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.
Answer:- Yes. Frying pan will vibrate when a stick is stuck against it. But we will not be able to listen the sound. Because sound does not travel into the vacuum.
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. They cannot talk with each other without using an special device. Because space is covered with an vacuum, and sound cannot travel without an medium. It does not travel in the vacuum.
20) List three sources of noise pollution in your locality.
Answer:- The sources of noise Pollution in our locality mainly contains,
Automobiles noise, loudspeakers and Dj, bands, bursting of crackers, television, mixer grinders, desert coolers. Etc.
Long Answer Questions
21) We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude then with a force of smaller magnitude. In which case would the instrument produce a louder sound?
Answer:-
Higher the Amplitude of the vibrations higher is the loudness of sound. Therefore the string plucked with the force of greater magnitude will produce higher Amplitude and thus the louder sound. Thus sound will be louder of force larger magnitude than the force of smaller magnitude.
22) How is sound produced and how is it transmitted and heard by us?
Answer:-
The listening device of our body is ear. Ear has an funnel like shape from the outer part. Inside it has an canal which is further goes to the eardrum which is the thin, stretched membrane. The sound enters the canal and moved at the end to the eardrum. These eardrum vibrate when there is vibration of sound, these vibration is sent to the inner ear by the eardrum. From the inner ear, the signal is sent to the brain. That these mechanism is called hearing. Thus by these method the sound produced, transmitted and heard by us.
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:- When the alarm clock is kept in the vessel that time man could hear the sound of alarm because sound can be travel through the vessel. but if the air inside the vessel is completely removed then there will be only vacuum, and sound cannot travel through vacuum. Hence the person standing over there cannot listen any sound after air removed completely.
24) The town hall 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:-
The level of sound or noise is low at the night time as compared to day time. As we know 70% of people do not work at the night, and there is quite silence. Day time is the busiest time of people and there is sound or noise everywhere. Therefore the sound of clock on the top of the town hall appears more clearly at the night time.
25) Suggest three measures to limit noise pollution in your locality.
Answer:-
1) We should spread an general awareness about the noise Pollution, so that people could understand the situation and keep control over the noise.
2) Tree plantation is must, because sound is absorbed by the trees and cannot move further.
We should plant trees near locality.
3) Vehicles horns should be keep with lower Amplitude.
4) Loudspeakers and bursting of crackers should be in control these is the major source of noise Pollution.