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Page No 170:

Question 1:

A scientist performed an experiment as shown in the picture here.
Figure

What do you think happened as air was pumped out of the jar and he rang the bell?
(a) The sound became louder.
(b) The sound became fainter first and then louder once all the air was pumped out.
(c) The sound could not be heard anymore.
(d) The sound was the same as before.

Answer:

(c) The sound could not be heard anymore.

 The sound could not be heard any more because it requires a medium for its propagation.

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Question 2:

Which of these pictures correctly show the way sound vibrations travel?
Figure

Answer:

 (c)

Sound vibrations travel in all possible directions. The correct option is (c).

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Question 3:

Which one of these rods of the wind chime produces the sound of highest pitch?
Figure

Answer:

Rod (a) will produce the sound of the highest pitch.

Short and thin objects produce sounds of a high pitch.

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Question 4:

Which of the following statements is correct?
(a) The time taken for one oscillation is the frequency.
(b) The distance covered by a vibrating body while making one complete oscillation is four times the amplitude.
(c) The greater the amplitude of a vibrating body, the higher will be the pitch of the sound it gives out.
(d) In a string instrument, thin strings are used for low pitch notes.

Answer:

(a) False

The time taken for one oscillation is the time period.

(b) False

The distance covered by a vibrating body while making one complete oscillation is its wavelength.

(c) False

The greater the amplitude of a vibrating body, the louder will be the sound it gives out.

(d) False

In a string instrument, thin strings are used for high-pitch notes.

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Question 5:

Rahul is playing the drums. Which of the following characteristics of sound would result in an increase in loudness?
(a) amplitude
(b) speed
(c) pitch
(d) quality

Answer:

 (a) Amplitude

Loudness depends upon amplitude.

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Question 6:

Each of the four bowls below is filled with water up to different levels. When struck with a wooden rod, which bowl will produce sound of the lowest pitch?
Figure

Answer:

Bowl (d) will produce the sound of the lowest pitch.

The vessel with the least water (most air) in it will produce the note of the lowest pitch.

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Question 7:

Science fiction films often show battles between spaceships. Four friends were arguing what would someone in space hear if an enemy spaceship exploded?
Rekha said, 'They would hear a big explosion.'
Renu said, 'They will not hear anything as there is no medium for sound to travel, but they can see the explosion.'
Ramesh said, 'Yes, they will not hear anything and they will not be able to see the explosion.'
Raghav said, 'The explosion produces sound of frequency above 20,000 Hz hence we cannot hear it.'
Who is correct?

(a) Rekha
(b) Renu
(c) Ramesh
(d) Raghav

Answer:

(b) Renu

A person in space will not hear anything as there is no medium for sound to travel. But the person will be able to see the explosion because light does not require any medium to travel.



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Question 8:

If the speed of sound in air is 330 m/s, the distance travelled by sound in 1/15th of a second is
(a) 20 m.
(b) 22 m.
(c) 30 m.
(d) 25 m.

Answer:

 (b) 22 m

Speed of sound in air = 330 m/s
Time = 1/15 sec
Distance travelled by sound = Speed × Time
                                                    = 330 m/sec × (1/15) sec
                                                    = 22 m

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Question 9:

What do you think a tabla player does to increase the pitch of the sound produced.
(a) He strikes the tabla skin harder.
(b) The pitch cannot be changed in a tabla.
(c) He tightens the skin by stretching the string around it.
(d) He gets a bigger tabla.

Answer:

(c) He tightens the skin by stretching the string around it.

A stretched membrane produces sound of a high pitch.

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Question 10:

The part of the ear that transmits sound vibrations to the brain is the
(a) cochlea.
(b) stirrup.
(c) eardrum.
(d) auditory nerve.

Answer:

(d) auditory nerve

The part of the ear that transmits sound vibrations to the brain is the auditory nerve.

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Question 11:

Put these terms, which describe the way sound travels from the air through your ear, in the correct sequence:
three tiny bones in the middle ear vibrate;your outer ear catches the sound and moves it inside your ear; a special nerve carries messages about sound to your brain; your eardrum begins to vibrate; liquid in the coiled tube of your inner ear vibrates

Answer:

Terms in the correct sequence:

Your outer ear catches the sound and moves it inside your ear; your eardrum begins to vibrate; the  liquid in the coiled tube of your inner ear vibrates; the three tiny bones in the middle ear vibrate; a special nerve carries messages about the sound to your brain.

 

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Question 12:

List the three properties of sound. What do these properties depend on?

Answer:

 

Property Property depends upon
Loudness Amplitude
Pitch Frequency
Quality The part of an instrument that is vibrating to produce sound

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Question 13:

Define frequency. What is its unit?

Answer:

Frequency is the number of vibrations produced in one second. It is measured in hertz (Hz).

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Question 14:

Mark the following statements true or false. Rewrite the false statements correctly.
(a) Sound has three properties-loudness, frequency and quality.
(b) A sound with large amplitude is louder than one with small amplitude.
(c) The loudness of sound is measured in hertz.
(d) Shortening the string of a guitar lowers its pitch.
(e) The eardrum vibrates when sound waves enter the ear.
(f) Frequency is the number of vibrations per second.
(g) Echoes are produced by the absorption of sound.
(h) Sound travels fastest in solids.
(i) The quality of sound produced by all stringed instruments will be the same.
(j) Radha tightens the string of her veena to raise the pitch of sound.

Answer:

(a) False
 
Sound has three properties: loudness, pitch and quality.
 

(b) True
 

(c) False
 
The loudness of sound is measured in decibels (db).
 

(d) False
 
Shortening the string of a guitar increases the pitch of its tone.
 
(e) True

(f) True

(g) False

Echoes are produced by the reflection of sound.

(h) True

(i) False

The quality of sound differs from one instrument to another.

(j) True

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Question 15:

Name one instrument in each case which makes musical notes by
(a) vibrating strings
(b) vibrating a piece of steel
(c) vibrating air in a tube

Answer:

(a) vibrating strings - Guitar
(b) vibrating a piece of steel - Cymbals
(c) vibrating air in a tube - Flute

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Question 16:

In each of these pictures point out in which case, A or B, the higher note is produced. Explain why.
Figure

Answer:

(a) A - A short pipe produces a higher note.

(b) A - A short string produces a higher note.

(c) B - A tight string produces a higher note.

(d) B - A short pipe produces a higher note.

(e) B - A short string produces a higher note.

(f) B - A thin rubber band produces a higher note.



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Question 17:

The deepest part of the oceans in our world is the Mariana Trench in the Pacific Ocean. It is around 11000 m deep. How long will it take for a signal sent from a ship above this trench to be reflected back? (The speed of sound in water = 1500 m/s.)

Answer:

Given: Speed of sound in water = v = 1500 m/s
Depth of the Mariana Trench in the Pacific Ocean = d = 11000 m
Let us assume that the time taken by a signal sent from a ship above the trench to be reflected back is t.
Total distance travelled by a signal = 2d
Time = Distance / speed
t = 2d / v
t = (2 × 11000) / 1500
t = 14.67 sec.
Thus, the time taken for a signal sent from a ship above the trench to be reflected back (t) is 14.67 sec.

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Question 18:

A boy stands at a distance of 165 m from a cliff. He claps his hand and hears an echo. If the speed of sound in air is 330 m/s, after how long does he hear the echo?

Answer:

Given: Distance between the boy and the cliff  = d = 165 m
Speed of sound in air  =  v  =  330 m/s
Let us assume that the time taken for the sound of the clap (echo) to reach the boy is t.
Total distance travelled by the sound = 2d
Time = Distance / Speed
t = 2d / v
t = (2 x 165) / 330
t = 1 sec
Thus, after one second, the boy will hear the echo.

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Question 19:

A mountaineer shouts for help. Half a second later she hears the echo. How far away is the rock which is reflecting her voice? (Speed of sound in air = 330 m/s.)

Answer:

Speed of sound in air = v = 330 m/s
Time (t) it took for the mountaineer to hear her echo = (1/2) sec
Let us assume that the distance between the mountaineer and the rock was d.
Distance = Speed × Time
2d = v × t
2d = 330 × (1/2)
d = 330 / 4 = 82.5 m
Thus, the rock that reflected her voice was 82.5 m away from her.

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Question 20:

Match the sounds in column I with the correct group in column II.

Column I Column II
a. a child whispering to another in class i. high pitch, loud sound
b. the whine of a mosquito near your ears ii. high pitch, soft sound
c. the bells of fire engines rushing past iii. low pitch, loud sound
d. the ticking of a clock iv. low pitch, soft sound

Answer:

Column I Column II
a. a child whispering to another in class iv. low pitch, soft sound
b. the whine of a mosquito near your ears ii. high pitch, soft sound
c. the bells of fire engines rushing past i. high pitch, loud sound
d. the ticking of a clock iii. low pitch, loud sound

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Question 21:

What effect can the following actions have in the sound of musical instruments?
(a) tightening the string
(b) shortening the air column
(c) increasing the length of the vibrating string
(d) plucking the string hard
(e) using a thick string

Answer:

(a) tightening the string - Pitch increases

(b) shortening the air column - Pitch increases

(c) increasing the length of the vibrating string - Pitch decreases

(d) plucking the string hard - Loudness increases

(e) using a thick string - Pitch decreases

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Question 22:

Answer the following questions briefly.
(a) 'A' vibrates 200 times in 5 s, while 'B' vibrates 35 times in 1 s. Which sound will have a lower pitch?
(b) Why do we need two ears?
(c) Why can't we hear the sound made by bats?
(d) When a bee flies, we hear the sound produced by its wings. Why can't we hear a crow flapping its wings as it flies?
(e) How can you raise the pitch of the note of a veena string without changing its length?
(f) The harder you beat a drum, the louder is the sound produced? Explain why this is so.
(g) Why are the walls and ceiling of a cinema hall covered with sound-absorbing material?

Answer:

(a) Pitch depends upon frequency also.
Frequency = (Number of vibrations) / (Time taken for the vibrations)

Frequency of A = 200 vibrations / 5 sec
                         = 40 Hz

Frequency of B = 35 vibrations / 1 sec
                          = 35 Hz
Thus, B has the lower pitch.
 
(b) We need two ears to locate a sound. If sound is produced on our left, the vibrations of the air reach our left ear just before they reach our right ear. They are heard more loudly in the left ear than in the right ear. The difference in the loudness of the sound in the two ears helps us to identify the direction the sound came from. 
 
(c) We cannot hear the sound made by bats because they produce ultrasonic sound of vibrations more than 20000 Hz, which is not audible to the human ear. 
 
(d) Human beings can hear a sound if its vibrations are within the frequency range of 20 Hz to 20 kHz. The sound vibrations produced by the wings of a flying bee are in this range, so this sound is audible to human. However, the sound produced by the flapping wings of a crow has a very low frequency of less than 20 Hz, so it is not audible to humans. 
 
(e) We can raise the pitch of a note of a Veena string by tightening the string. 
 
(f) The stretched skin on the top of a drum vibrates when it is struck. When we beat a drum harder, vibrations of a large amplitude are produced. The loudness of sound depends on the amplitude of the vibrations. Sounds of large amplitudes are loud.  
 
(g) The walls and ceiling of a cinema hall are covered with sound-absorbing material to prevent the reflection of sound, or echo.



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