ICSE Solutions for Chapter 8 Propagation of Sound Waves Class 9 Physics Selina Publisher

Exercise -8(A)

1. What causes sound?
Answer 

Sound is caused due to vibrations of a body.


2. What is sound? How is it produced?
Answer 

Sound is a form of energy that produces the sensation of hearing in our ears. Sound is produced by a vibrating body.


3. Complete the following sentence:

Sound is produced by a _____ body.

Answer 

Vibrating


4. Describe a simple experiment which demonstrates that the sound produced by a tuning fork is due to vibrations of its arms.
Answer 

Experiment: A tuning fork is taken and its one arm is struck on a rubber pad and it is brought near a tennis ball suspended by a thread as shown in figure.

It is noticed that as the arm of the vibrating fork is brought close to the ball, it jumps back and forth and sound of the vibrating tuning fork is heard. When its arm stop vibrating, the ball becomes stationary and no sound is heard.


5 . Describe in brief, with the aid of a sketch diagram, an experiment to demonstrate that a material medium is necessary for propagation of sound.
Answer 

Experiment to demonstrate that a material medium is necessary for the propagation of sound:

An electric bell is suspended inside an airtight glass bell jar. The bell jar is connected to a vacuum pump as shown in figure. As the circuit of electric bell is completed by pressing the key, the hammer of the electric bell begins to strike the gong repeatedly due to which sound is heard.

Keeping the key pressed, air is gradually withdrawn from jar by starting the vacuum pump. It is noticed that the loudness of sound goes on decreasing as the air is taken out from the bell jar and finally no sound is heard when all the air from the jar has been drawn out. The hammer of the electric bell is still seen striking the gong repeatedly which means that sound is still produced but it is not heard.

When the jar is filled with air, the vibrations produced by the gong are carried by the air to the walls of jar which in turn set the air outside the jar in vibration and sound is heard by us but in absence of air, sound produced by bell could not travel to the wall of the jar and thus no sound is heard. It proves that material medium is necessary for the propagation of sound waves.


6. There is no atmosphere on moon. Can you hear each other on the moon’s surface?
Answer 

We cannot hear each other on moon’s surface because there is no air on moon and for sound to be heard, a material medium is necessary.


7. State three characteristics of the medium required for propagation of sound?
Answer 

Requisites of the medium for propagation of sound:

  • The medium must be elastic.
  • The medium must have inertia.
  • The medium should be friction less.


8. Explain with an example, the propagation of sound in a medium.
Answer 

Take a vertical metal strip with its lower end fixed and upper end being free to vibrate as shown in fig (a).

As the strip is moved to right from a to b as shown in Fig (b), the air in that layer is compressed (compression is formed at C). The particles of this layer compress the layer next to it, which then compresses the next layer and so on. Thus, the disturbance moves forward in form of compression without the particles themselves being displaced from their mean positions.

As the metal strip returns from b to a as shown in Fig (c) after pushing the particles in front, the compression C moves forward and particles of air near the strip return to their normal positions.

When the strip moves from a to c as shown in Fig (d), it pushes back the layer of air near it towards left and thus produces a low pressure space on its right side i.e. layers of air get rarefied. This region is called rarefaction (rarefaction is formed at R).

When the strip returns from C to its mean position A in Fig (e), the rarefaction R travels forward and air near the strip return to their normal positions.

Thus, one complete to and fro motion of the strip forms one compression and one rarefaction, which together form one wave. This wave through which sound travels in air is called longitudinal wave.


9. Choose the correct word/words to complete the following sentence:

When sound travels in a medium _______ (the particles of the medium, the source, the disturbance, the medium) travels in form of a wave.

Answer 

When sound travels in a medium the disturbance travels in form of a wave.


10. Name the two kinds of waves in form of which sound travels in a medium.
Answer 

Sound travels in a medium in form of longitudinal and transverse waves.


11. What is a longitudinal wave? In which medium: solid, liquid or gas, can it be produced?
Answer 

A type of wave motion in which the particle displacement is parallel to the direction of wave propagation is called a longitudinal wave. It can be produced in solids, liquids as well as gases.


12. What is a transverse wave? In which medium: solid, liquid or gas, can it be produced? 
Answer 

A type of wave motion in which the particle displacement is perpendicular to the direction of wave propagation is called a transverse wave. It can be produced in solids and on the surface of liquids.


13. Explain meaning of the terms compression and rarefaction in relation to a longitudinal wave. 
Answer 

A longitudinal wave propagates by means of compression and rarefaction.

When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in Fig. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R), as shown in Fig.

Compressions are the regions of high density where the particles of the medium come very close to each other and rarefactions are the regions of low density where the particles of the medium move away from each other.


14. Explain the terms crest and trough in relation to a transverse wave. 
Answer 

A crest is a point on the transverse wave where the displacement of the medium is at a maximum.

A point on the transverse wave is a trough if the displacement of the medium at that point is at a minimum.


15. Describe an experiment to show that in wave motion, only energy is transferred, but particles of medium do not move.
Answer 

Experiment to show that in a wave motion, only energy is transferred, but particles of the medium do not move:

If we drop a piece of stone in the still water of pond, we hear a sound of stone striking the water surface. Actually a disturbance is produced in water at the point where the stone strikes it. This disturbance spreads in all directions radially outwards in form of circular waves on the surface of water.

If we place a piece of cork on water surface at some distance away from the point where the stone strikes it, we notice that cork does not move ahead, but it vibrates up and down, while the wave moves ahead. The reason is that particles of water (or medium) start vibrating up and down at the point where the stone strikes. These particles then transfer their energy to the neighboring particles and they themselves come back to their mean positions. Thus only energy is transferred but the particles of the medium do not move.


16. Define the term amplitude of a wave. Write its S.I. unit. 
Answer 

The maximum displacement of the particle of medium on either side of its mean position is called the amplitude of wave.

Its SI unit is metre.


17. What do you mean by the term frequency of a wave? State its S.I. unit. 
Answer 

The number of vibrations made by the particle of the medium in one second is called the frequency of the wave. It can also be defined as the number of waves passing through a point in one second.

Its SI unit is hertz (Hz).

饾湀 = 1/T


18. How is the frequency of a wave related to its time period?
Answer 

Frequency of a wave is the reciprocal of the time period.


19. Define the term wave velocity. Write its S.I. unit. 
Answer 

The distance travelled by a wave in one second is called its wave velocity.

Its SI unit is metre per second (ms-1).


20. Draw displacement-time graph of a wave and show on it the amplitude and time period of wave. 
Answer 


21. Draw a displacement-distance graph of a wave and mark on it, the amplitude of wave by the letter ‘a’ and wavelength of wave by the letter. 
Answer 


22. How are the wave velocity v, frequency and wavelength of a wave related? Derive the relationship. 
Answer 

Let the velocity of a wave be v, time period T, frequency V and wavelength 位 . By the definition of wavelength, 
Wavelength = Distance travelled by the wave in one time period i.e., in T second 
Or, wavelength = Wave velocity × Time period 
Or, 位 = v × T
or, 位  = v × 1/f  
Therefore, V = v位
Therefore, Wave velocity  = Frequency × wavelength 


23. State two properties of medium on which the speed of sound in it depends. 
Answer 

The speed of sound in a medium depends upon its elasticity and density.


24. Arrange the speed of sound in gases Vg, solids Vs and liquids VL  in an ascending order.     
Answer 

VL < VL < Vs       


25. State the speed of (i) light and (ii) sound in air?
Answer 

(i) Speed of light in air = 3×108 m s-1 

(ii) Speed of sound in air = 330 m s-1.


26. Compare approximately the speed of sound in air, water and steel. 
Answer

1 : 4 : 15


27. Answer the following questions:

(i) Can sound travel in vacuum?

(ii) How does the speed of sound differ in different media?

Answer 

(i) No, sound cannot travel in vacuum as it requires a material medium for its propagation.

(ii) Speed of sound is maximum in solids, less in liquids and least in gases.


28. Flash of lightning reaches earlier than the sound of thunder. Explain the reason. 
Answer 

This happens because the light travels much faster than sound.


29. If you place your ear close to an iron railing which is tapped some distance away, you hear the sound twice. Explain why?
Answer 

Sound travels in iron faster than in air so first the sound travelled in iron rail is heard and then the sound travelled in air is heard.


30. The sound of an explosion on the surface of a lake is heard by a boat man 100 m away and by a diver 100 m below the point of explosion.

(i) Who would hear the sound first: boatman or diver?

(ii) Give a reason for your answer in part (i).

(iii) If sound takes time to reach the boatman, how much time approximately does it take to reach the diver?

Answer 

(i) The diver would hear the sound first.

(ii) This is because sound travels faster in water than in air.

(iii) It would take 0.25t to reach the diver because sound travels almost four times faster in water.


31. How do the following factors affect, if at all, the speed of sound in air:

(i) Frequency of sound,

(ii) Temperature of air,

(iii) Pressure of air and

(iv) Moisture in air?

Answer 

(i) Frequency of sound has no effect on the speed of sound.

(ii) Speed of sound increases with the increase in the temperature of sound.

(iii) Pressure of sound has no effect on the speed of sound.

(iv) Speed of sound increases with the increase in presence of moisture in air.


32. How does the speed of sound change with change in (i) amplitude and (ii) wavelength, of sound wave?
Answer 

(i) Speed of sound does not change with a change in amplitude.

(ii) Speed of sound does not change with a change in wavelength.


33. In which medium the speed of sound is more: humid air or dry air? Give a reason to your answer.
Answer 

Speed of sound is more in humid air because in presence of moisture, the density of air decreases and sound travels with greater speed.

v ∝ 1/√蟻


34. How does the speed of sound in air vary with temperature? Answer 

The speed of sound increases by 0.61 m s-1 for each 1 rise in temperature.


35. Describe a simple experiment to determine the speed of sound in air. What approximation is made in the method described by you? Answer

The simple experiment that a person can do to calculate the speed of sound in air is that a person stands at a known distance (d meter) from the cliff and fires a pistol and simultaneously start the stopwatch. He stops the stopwatch as soon as he hears an echo. The distance travelled by the sound during the time (t) seconds is 2d. So, speed of sound = distance travelled / time taken = 2d/t

The approximation made is that speed of sound remains same for the time when the experiment is taking place.


36. Complete the following sentences :

(a) Sound cannot travel through ______, but it requires a ______.

(b) When sound travels in a medium, the particles of medium _____ but the disturbance ______.

(c) A longitudinal wave is composed of compression and ______.

(d) A transverse wave is composed of crest and _____.

(e) Wave velocity = ______ × wavelength.

Answer 

(a) Vacuum, medium

(b) do not move, moves

(c) rarefaction

(d) trough

(e) frequency


Multiple Choice Questions -8(A)

1. The correct statement is :

(a) Sound and light both require medium for propagation.

(b) Sound can travel in vacuum, but light can not

(c) Sound needs medium, but light does not need medium for its propagation.

(d) Sound and light both can travel in vacuum.

Answer 

(c) Sound needs medium, but light does not need medium for its propagation.


2. The speed of sound in air at 0C is nearly:

(a) 1450 m s-1 

(b) 450 m s-1

(c) 5100 m s-1

(d) 330 m s-1

Answer 

(d) 330 m s-1


3. Sound in air propagates in form of

(a) Longitudinal wave

(b) Transverse wave

(c) Both longitudinal and transverse waves

(d) Neither longitudinal nor transverse wave.

Answer

(a) Longitudinal wave


4. The speed of light in air is :

(a) 3×108 m s-1 

(b) 330 m s-1

(c) 5100 m s-1

(d) 3 × 1010 m s-1

Answer 

3×108 m s-1


Numerical -8(A)

1. The heart of a man beats 75 times a minute.

What is its (a) frequency and (b) time period?

Answer 

Given, heart beats 75 times a minute

(a) Frequency = No. of times heart beats in 1 s

Or, 饾湀 = 75/60 = 1.25 s-1

(b) Time period, T = 1/饾湀

Or, T = 1/1.25 = 0.8 s


2. The time period of a simple pendulum is 2 s. Find its frequency. 
Answer 

Frequency, 饾湀 = 1/T

Or, 饾湀 = 1/ 2 = 0.5 Hz


3. The separation between two consecutive crests in a transverse wave is 100 m. If wave velocity is 20 m s-1, find the frequency of wave. 
Answer 

Given, wavelength = 100m

Wave velocity = 20 m/s

We know that,

Wave velocity = Frequency × Wavelength

Or, Frequency = Wave velocity / wavelength

Or, 饾湀 = 20/100 = 0.2 Hz


4. A longitudinal wave travels at a speed of 0.3 m s-1 and the frequency of wave is 20 Hz. Find the separation between two consecutive compressions. 
Answer 

Wave velocity = 0.3 m/s

Frequency = 20 Hz

Separation between two consecutive compressions is the wavelength of a wave.

We know that,

Wave velocity = Frequency x Wavelength

Or, wavelength = Wave velocity/frequency

Or, 位 = 0.3/20 = 1.5 ×10-2 m


5. A source of wave produces 40 crests and 40 troughs in 0.4 s. What is the frequency of the wave?
Answer 

Frequency of wave = number of waves per second

Or, v = 40/0.4 = 100 Hz


6. An observer fires a gun and another observer at a distance 1650 m away from hears its sound. If the speed of sound is 330 m s-1, find the time when will hear the sound after firing by A. 
Answe

Distance between the two observers = 1650 m

Speed of sound = 330 m/s

Time in which B hears the sound = Distance/speed = 1650/330 = 5s

Thus, B will hear the sound 5s after the gun is shot.


7. The time interval between a lightning flash and the first sound of thunder was found to be 5 s. If the speed of sound in air is 330 m s-1, find the distance of flash from the observer.
Answer 

Speed of sound in air (v) = 330 m/s

Time in which thunder is heard after lighting is seen (t) = 5s

Thus, distance between flash and observer = V× t = (330×5) = 1650 m


8. A boy fires a gun and another boy at a distance hears the sound of fire 2.5s after seeing the flash. If the speed of sound in air 340 m/s, find distance between the boys.

Answer 

Speed of sound in air (v) = 340 m/s

Time in which sound of fire is heard after flash is seen (t) = 2.5s

Thus, distance between flash and observer = v× t = (340 ×2.5) = 850 m


9. An observer sitting in line of two tanks watches the flashes of two tanks firing at each other at the same time, but he hears the sounds of two shots 2s and 3.5s after seeing the flashes. If distance between the two tanks is 510m, find the speed of sound. 
Answer 

Time taken by the observer to hear the sound of the first tank A= 3.5s

Now Time taken by the observer to hear the sound of the second tank B = 2s

This Time taken by the tank B to hear the sound of tank A= (3.5 – 2)s = 1.5s

Distance between the two tanks = 510m

Speed = 510/1.5=340m/s


10. How long will sound take to travel in (a) and iron rail and (b) air, both 3.3 km in length? Take speed of sound in air to be 330 m/s and in iron to be 5280 m/s. 
Answer 

(a) Length of iron rail (D) = 3.3 km = 3300 m

Speed of sound in iron (V) = 5280 m/s

Time taken by sound to travel in iron rod (t) = D/V

Or, t = (3300 / 5280) s = 0.625 s

(b) Length of iron rail (D) = 3.3 km = 3300 m

Speed of sound in air (V) = 330 m/s

Time taken by sound to travel in iron rod (t) = D/V

Or, t = (3300/330) s = 10 s


11. Assuming the speed of sound in air equal to 340 m/s and in water equal to 1360 m/s, find the time taken to travel a distance 1700 m by sound in (i) air (ii) water. 
Answer 

(i) Distance travelled (D) = 1700

Speed of sound in air (V) = 340 m/s

Time taken (t) = D/V = (1700 / 340) s = 5 s

(ii) Distance travelled (D) = 1700

Speed of sound in water (V’) = 1360 m/s

Time taken (t) = D/V = (1700 / 1360) s = 1.25 s


Exercise -8(B)

1. What do you mean by the audible range of frequency?
Answer 

The range of frequency within which the sound can be heard by a human being is called the audible range of frequency.


2. What is the audible range of frequency for human?
Answer 

The audible range of frequency for humans is 20 Hz to 20 kHz.


3. For which range of frequencies, human ears are most sensitive?
Answer 

Human ears are most sensitive for the range 2000 Hz to 3000 Hz.


4. Which has the higher frequency - ultrasonic sound or infrasonic sound?
Answer 

Ultrasonic has higher frequency.


5. Complete the following sentences:

(a) An average person can hear sound of frequencies in the range _____ to _____.

(b) Ultrasound is of frequency ______.

(c) Infrasonic sound is of frequency ______.

(d) Bats can produce and hear ______ sound.

(e) Elephants produce ____________sound.

Answer 

(a) 20 Hz, 20 kHz

(b) above 20 kHz

(c) below 20 Hz

(d) ultrasonic

(e) infrasonic.


6. Name the sounds of the frequencies given below:

(a) 10 Hz

(b) 100 Hz

(c) 1000 Hz

(d) 40 kHz

Answer 

(a) Infrasonic

(b) Audible

(c) Audible

(d) Ultrasonic.


7. Can you hear the sound produced due to vibrations of a seconds pendulum? Give reasons. 
Answer 

No, we cannot hear the sound produced due to vibrations of a seconds pendulum because the frequency of sound produced due to vibrations of seconds pendulum is 0.5 Hz which is infrasonic.


8. What is ultrasound?
Answer 

Sounds of frequency above 20 kHz are called ultrasound.


9. State the approximate speed of ultrasound in air.
Answer 

The approximate speed of ultrasound in air is 330 m/s-1.


10. State two properties of ultrasound that make it useful to us. 
Answer 

Two properties of ultrasound which make it useful to us are:

  • High energy contents
  • High directivity


11. Explain how do bats locate the obstacles and prey in their way.
Answer 

Bats locate the obstacles and prey in their path by producing and hearing the ultrasound. They emit an ultrasound which returns after striking an obstacle in their way. By hearing the reflected sound and from the time interval (when they produce ultrasound and they receive them back), they can judge the direction and the distance of the obstacle in their way.


12. State two applications of ultrasound. 
Answer 

Two applications of ultrasound:

(i) Ultrasound is used for drilling holes or making cuts of desired shape in materials like glass.

(ii) Ultrasound is used in surgery to remove cataract and in kidneys to break the small stones into fine grains.


Multiple Choice Questions -8(B)

1. A man can hear the sound of frequency :

(a) 1 Hz

(b) 1000 Hz

(c) 200 kHz

(d) 5 MHz

Answer 

(b) 1000 Hz


2. The properties of ultrasound that make it useful are

(a) High power and high speed

(b) High power and good directivity 

(c) High frequency and high speed

(d) High frequency and bending around the objects.

Answer 

(b) High power and good directivity


3. Sonar makes use of :

(a) infrasonic sound

(b) ultrasound

(c) ordinary sound

(d) light

Answer 

(b) ultrasound

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