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CBSE Questions for Class 8 Physics Sound Quiz 9 - MCQExams.com
CBSE
Class 8 Physics
Sound
Quiz 9
The time period of a sound wave from a piano is $$1.18\times10^{-3} s$$. Find its frequency.
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$$847.45 Hz$$
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$$800 Hz$$
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$$935. 55 Hz$$
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$$1000 Hz$$
Explanation
The frequency (f) of a wave is the number of full waveforms generated per second. This is the same as the number of repetitions per second or the number of oscillations per second.
Time Period (T) is the number of seconds taken to cover one wavelength or the number of seconds per oscillation. It is clear that frequency and period are reciprocals.
In this case, the time period is given as $$1.18\times { 10 }^{ -3 }s$$.
So, the frequency $$f=\frac { 1 }{ T } =\frac { 1 }{ 0.00118s } =847.45Hz.$$
A tuning fork's tongs vibrate 250 times in 2.0s. Find the frequency of vibration
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125 Hz
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200 Hz
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250 Hz
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600 Hz
Explanation
The frequency (f) of a wave is the number of full waveforms generated per second. This is the same as the number of repetitions per second or the number of oscillations per second.
Time Period (T) is the number of seconds per waveform or the number of seconds per oscillation. It is clear that frequency and period are reciprocals.
In this case, A tuning fork's tongs vibrate 250 times in 2.0 s.
That is, the frequency is given as $$f=\dfrac { 250 }{ 2 } =125 Hz$$.
Time period = $$\dfrac { 1 }{ f } =\dfrac { 1 }{ 125 } $$=$$8\times { 10 }^{ -3 }s$$.
Hence, the frequency is 125 Hz and the time period is
$$8\times { 10 }^{ -3 }s$$
seconds
.
Which of the following pictures correctly show the way sound vibrations travel?
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Explanation
The answer is C.
When a body vibrates, the sound wave produced by it will travel in all directions. The third figure in the option depicts that the sound is travelling in all directions. Therefore, this figure correctly represents the travelling of sound.
Take a metallic tumbler and a tablespoon. Strike the tablespoon gently at the brim of the tumbler (as shown in the figure above). Now suspend a small thermocole ball touching the rim of the tumbler and touch it to the vibrating tumbler. See how far the ball is displaced. Now, again strike the tablespoon at the brim of the tumbler but strike it hard this time. Again touch the thermocole to the vibrating tumbler and see how far the ball is displaced. Choose the correct option.
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Thermocole ball is displaced more when the tumbler is striked hard, as frequency if vibration is more in this case.
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The thermocole ball is displaced less when the tumbler is striked hard, as frequency of vibration is less in this case.
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The thermocole ball is displaced more then the tumbler is striked hard, as amplitude of vibration is more in this case.
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The thermocole ball is displaced more when the tumbler is striked hard, as amplitude of vibration is less in this case.
Explanation
In our observation, we see that the thermocole ball is displaced more when the tumbler is stricken hard, as the amplitude of vibration is more in this case.
The loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel (dB).
The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.
Hence, option C is correct.
Calculate the frequency, in hertz, for 120 Oscillations in 2.0 s
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60 Hz
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100 Hz
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80 Hz
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120 Hz
Explanation
The frequency (f) of a wave is the number of full waveforms generated per second. This is the same as the number of repetitions per second or the number of oscillations per second.
Time Period (T) is the number of seconds per waveform or the number of seconds per oscillation. It is clear that frequency and period are reciprocals.
In the first case, there are 120 Oscillations in 2.0 s.
That is, the frequency is given as $$f=\dfrac { 120 }{ 2 } =60Hz$$.
Calculate the frequency for ten swings of a pendulum in $$6.7\ s$$.
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$$1.492\ Hz$$
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$$0.555\ Hz$$
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$$2.372\ Hz$$
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$$4.864\ Hz$$
Explanation
The number of complete oscillations per unit time is called
frequency.
Given,
Number of swings/oscillations $$= 10$$
Time for $$10$$ oscillations, $$t=6.7\ s$$
Frequency, $$f=\dfrac{Number\ of\ oscillations}{time}$$
Frequency, $$f=\dfrac{10}{6.7}$$
$$f=1.492\ Hz$$
A scientist performed an experiment as shown in the picture above. What happened as air was pumped out of the jar and he rang the bell?
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The sound became louder.
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The sound became fainter first and then louder once all the air was pumped out.
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The sound could not be heard anymore.
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The sound was the same as before.
Explanation
Sound waves need medium to propagate from one point to another such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter. The molecules in solids are packed very tightly. Liquids are not packed as tightly as solids. And gases are very loosely packed. The spacing of the molecules enables sound to travel much faster through a solid than a gas. Sound travels about four times faster and farther in water than it does in air.
When air is pumped out from the jar by this process, vacuum will creat inside the jar which not allow to propagate sound wave.
Hence, when the bell rings, sound cannot be heard by anyone. (Option C)
The part of the violin which vibrates while playing it is :
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The wire
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The box of the violin
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Both the wire and the box
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None of the above
Explanation
Drawing the bow causes the string to vibrate. These vibrations are transferred to the bridge, the front, the soundpost, and the back of the violin. The air inside the hollow body of a violin vibrates at the same frequency.
The time
period
of a vibrating body is $$0.01\;sec$$. Its frequency will be :
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$$1.0\;s^{-1}$$
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$$10.0\;s^{-1}$$
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$$100.0\;s^{-1}$$
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$$1000.0\;s^{-1}$$
Explanation
Given,
Time period $$=0.01\,sec$$
We have to find the frequency.
We have the relation,
$$Frequency=\dfrac{1}{Time\,Period}$$
$$\therefore Frequency=\dfrac{1}{0.01}=100\,s^{-1}$$
Which of the following figures represents a loud sound of low pitch ?
Where y = displacement (m), t = time (s)
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A sound wave completes $$20$$ vibrations in $$2.5\;s$$. Its frequency will be equal to :
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$$20\;Hz$$
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$$8\;Hz$$
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$$200\;Hz$$
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$$50\;Hz$$
Explanation
We have,
$$Frequency=\frac{Number\,of\,vibration}{Time\,taken}$$
It is given that the sound wave completes 20 vibrations in 2.5 seconds
Hence,
$$Frequency=\frac{20}{2.5}=8\,Hz$$
A body produces sound only if it is :
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made of steel
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made of glass
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plucked
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vibrating
Explanation
Sound is produced by the vibration of any object in a medium. If the vibration is enough for humans to percieve and the vibration is done within a medium around, it will produce sound. If the vibrating body are in space, they will not produce sound because there is no medium present.
There is no atmosphere on moon. Therefore,
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no sound can be heard
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sound can be easily heard
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sound is sometimes heard
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all the above statements are correct
Explanation
Since sound waves are longitudinal in nature, so it requires a material medium to propagate. As there is no atmosphere on moon, so sound wave cannot propagate on moon. Thus, sound cannot be heard on moon.
Consider the following sounds: G -conversation between two girls, P-sound produced by a peacock and M-sound produced by a moving motorcycle. These sounds in the increasing order of frequency are
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G, P, M
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P, M, G
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P, G, M
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M, G, P
Explanation
The Pitch of sound depends on the frequency. We know that the sound produced by a motorcycle has the lowest pitch and the sound produced by a peacock has the highest pitch.
Thus the arrangement in the increasing order of frequency is -
$$\text{Motor Cycle}<\text{Conversation between two girls}<\text{Peacock}$$
$$\therefore M, G, P$$
A person is listening to a tone of $$500 \ Hz$$ sitting at a distance of $$450 \ m$$ from the source of the sound. What is the time interval between the successive compression from the source?
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$$5 \ ms$$
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$$1 \ ms$$
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$$2 \ ms$$
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$$2 \ s$$
Explanation
Given - $$ n=500 \ Hz$$ ,
The time interval between two successive compressions or rarefactions is called , time period of wave , which is given by ,
$$T=\dfrac 1n= \dfrac{1}{500}= 0.002 \ s=2 \ ms$$
A person is listening to a tone of $$500$$ Hz sitting at a distance of $$450$$ m from the source of the sound. What is the time interval between the successive compression from the source ?
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$$5$$ milli second
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$$1$$ milli second
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$$2$$ milli second
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$$2$$ second
Explanation
Given - $$ f=500Hz$$ ,
The time interval between two successive compressions or rarefactions is the
time period
of the wave , which is given by ,
$$T=1/f=1/500=0.002s=2\ milli\ second$$
The product of the time period of a sound wave and its frequency is equal to:
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infinite
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zero
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more than unity but less than infinity
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unity
Explanation
We have the equation,
$$f=\frac{1}{T}$$
where,
$$f$$ is the frequency
$$T$$ is the time period
Hence the product of the time period of the sound wave and its frequency is equal to unity.
The vibrating body while playing a violin is
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Air
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The box
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Both wire and box
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Air, wire and the box
Explanation
The sound of a violin is the result of interactions between its many parts. Drawing a bow across the strings causes the strings to vibrate. This vibration is transmitted through the bridge
and sound post to the body of the violin, which allows the sound to effectively radiate into the surrounding air. So the vibrating body while playing a violin is both wire and box.
The frequency of a wave is 5 Hz. It refers to (type of wave)
___________.
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ultrasonics
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microwaves
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infrasonics
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radio waves
Explanation
The waves below the frequency of 20 Hz are infrasonic waves. So, the wave of 5 Hz is also an infrasonic wave.
Only vibrating bodies produce sound. State true or false
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True
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False
Explanation
Sound is produced when something vibrates. The vibrating body causes the medium around it to vibrate. Vibrations in air are called travelling longitudinal waves, which we can hear.
A normal man is unable to hear the sound emitted by source. Out of the possible reasons, the one which is not applicable is
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The pitch of the sound may be too high
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The pitch of the sound may be too low
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The quality of the sound is not matching with the man's requirement
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The amplitude of sound may be too small
Arwin is playing violin and his sister Tina is listening music from the loudspeaker. Which one of the two children is creating more noise pollution as compare to other?
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Arwin
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Tina
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Both Arwin and Tina
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Neither Arwin nor Tina
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None of these
Explanation
Sound of violin is pleasant but sound of a loudspeaker is always noise because it is at high volume. Therefore, option (B) is correct and rest of the options is incorrect.
Every source of sound has to be a ________ body.
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vibrating
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resting
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climbing
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elastic
Explanation
Every source of sound has to be a vibrating body. When a body vibrates, it forces the neighbouring particle of the medium to vibrate. This creates a disturbance in the medium, which travels in the form of waves. This disturbance, when reaches the ear, produces sound.
Imagine a cannon being fired on the surface of the moon. Then
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the sound will be heard at the surface of the earth during all seasons
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the sound will not be heard at the surface of the earth
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the sound will be heard at the surface of the earth during the rainy season
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no sound will be heard on the earth or on the moon
Explanation
Sound waves need a material medium to propagate. Since there is no atmosphere on the moon, so sound cannot propagate in space and hence no sound will be heard at the surface of the earth as well as on the moon.
Which of the following statements is incorrect?
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Sound needs medium to travel
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Sound can travel through vaccum
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Sound can't travel through vaccum
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Both A and B
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None of these
Explanation
Sound needs a medium to travel so that the molecules of medium can transfer sound vibrations.Sound can't travel through vacuum as there are no molecules or matter in vaccum. Therefore, option (B) is incorrect.
Not all kinds of vibration in the material bodies produce sensation of sound. True or false
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True
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False
Explanation
All kinds of vibration in the material bodies do produce sensation of sound
Which of the following will produce sound through vibration?
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a plastic scale when bent over the edge of a bench
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saying "Ah" for few seconds
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plucking a rubber string by fingers
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all
Explanation
We know that sound is produced whenever there is any vibration.
All vibration produces sound. In some cases vibrations are visible to our naked eyes but in some cases they can only be felt not seen.
In all the above activities , there is some kind of vibration involved. So sound can be produced.
For example.
Plastic scale when bent and left again vibrates rapidly and saying 'Ah' from mouth involves vibration of vocal cord.And plucking a rubber string vibrates the string rapidly.So all these cases produce sound.
When a flute is blown, the air inside it gets compressed and produces sound but does not vibrate. True or false.
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True
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False
Explanation
The flute is an instrument that works on the theory of an open cylindrical air column. Sound is produced from a flute by blowing onto a sharp edge, causing air enclosed in the tube to get compressed and vibrate.
State whether true or false.
The other name for the voice box in humans is Larynx.
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True
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False
Explanation
Larynx is the portion of the respiratory tract containing the vocal cords which produce sound. It is located between the pharynx and the trachea. The larynx is also called as the voice box.
When we hit a thin plastic ruler strongly on a bench:
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the free end of the ruler vibrates
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the free end of the ruler produces sound
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even after it stops vibrating it continues to produce sound
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Both A and B
Explanation
When a ruler hits strongly on the bench, the free end vibrates up and down and produces sound.
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