FRANK Solutions for Class 9 Physics Chapter 7 - Sound

FRANK Solutions for Class 9 Physics Chapter 7 - Sound

Chapter 7 - Sound Exercise 286

Question 1

Can you hear each other in a dark room?

Solution 1

Yes, we can hear in the dark room.

Question 2

Does the speed of sound in a gas depend on the pressure of the gas?

Solution 2

No, the speed of sound in the gas is independent on the change in pressure of the gas.

Question 3

Write the formula for speed of sound in a gas using Laplace correction.

Solution 3

Question 4

Does the speed of sound depend upon the direction of wind?

Solution 4

Yes, the speed of sound depends on the direction of wind. When wind is blowing in the direction of propagation of sound , the speed of sound increases. But, when wind blows in the direction opposite to that of sound , the speed of sound decreases.

Question 5

What are ultrasonic?

Solution 5

Ultrasonic are sounds having frequency higher that 20 KHz and not audible to the human being.

Question 6

What does SONAR stand for?

Solution 6

SONAR stands for the "sound navigation and ranging".

Question 7

Which has the higher frequency: ultrasonic sound or infrasonic sound?

Solution 7

Ultrasonic sound has higher frequency than infrasonic sound.

Question 8

Can you hear the sound produced due to the vibrations of second's pendulum?

Solution 8

No, we cannot hear the sound produced due to the vibration of second's pendulum.

Question 9

Does SONAR make use of infrasonic sound?

Solution 9

No, SONAR does not make use of infrasonic waves. Instead, it make use of ultrasonic waves.

Question 10

Does ultrasonic in a medium travel faster than the audible sound?

Solution 10

Yes, ultrasound in the medium travels faster than the audible sound.

Question 11

Is it possible to hear your friend's voice in vacuum? Give reason for your answer.

Solution 11

No, it is not possible to hear the friend's voice in vacuum because sound needs a material medium to travel.

Question 12

What causes sound?

Solution 12

Sound is produced by the vibrating objects.

Question 13

What are the requisites of the medium for the propagation of sound? Briefly, explain them.

Solution 13

The following are the requisites of medium to travel:
(i) The medium should possess elasticity so that particles of medium have tendency to come back to their original position after the displacement.
(ii) The medium should be frictionless so that there should be loss of energy.
(iii) The medium must possess inertia so that particles of medium have ability to store energy.

Question 14

What cause sound? Why do we require a material medium for the transmission of sound?

Solution 14

Sound is produced by the vibrating objects. We require a material medium for the propagation of sound because vibrations of object need to get transferred or transmitted from one place to another.

Question 15

What is the effect of temperature and pressure on the speed of sound in a gas?

Solution 15

With increase in temperature of gas, there is increase in speed of gas because increase in temperature decreases the density of gas. And speed of sound is inversely proportional to density of gas. The speed of gas is independent of the pressure of gas.

Question 16

(a) What is the effect of wavelength and amplitude on the speed of sound?
(b) Explain the effect of density on the speed of sound.

Solution 16

(a) No, there is no effect of wavelength and amplitude on the speed of sound.
(b) With increase in density, there is decrease in the speed of sound in gas.

Question 17

What is sound? Explain with example how does sound travel in a medium.

Solution 17

Sound is defined as form of energy that produces the sensation of hearing in our ears. The sound travels in medium through energy transfers by wave motion from one place to another. For example , when the prong of the tuning fork vibrate , the waves start moving from one fork to another , the air start getting compressed . The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbor which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.

Question 18

Compare approximately the speed of sound in air and steel.

Solution 18

The ratio of speed of sound in air to speed of sound in steel is 1/15.

Question 19

The smoke from the gun barrel is seen 2 second before the explosion is heard. If the speed of sound in air is 350 ms^-1, calculate the distance of observer from the gun.

Solution 19

The distance of observer from the gun = speed of sound x time taken for the explosion
= 350 x 2 = 700m

Question 20

Does light travel faster in air than in water? Why?

Solution 20

Yes, light travels faster in air than water. Because, light does not require any medium for their propagation and they are not mechanical waves. The quantities which oscillate in light are mechanical and electric field. And electric and magnetic field decrease with the medium in between them. So, light travels faster in air than water.

Question 21

Why does the flash of lighting reach earlier than the sound of thunder?

Solution 21

The flash of light reaches earlier than the sound of thunder because the speed of light (its 3 x 10⁸) is much larger than the speed of sound in air (its 330 m/s).

Question 22

The sound of which of the following frequency is audible to us?
2 Hz, 1000 Hz, 200 kHz, 8 MHz

Solution 22

The sound of 1000 Hz frequency will be audible to us. Others 2Hz is infrasonic sound frequency and 200 KHz, 8 MHz is ultrasonic sound frequency.

Question 23

Is sound a form of energy?

Solution 23

Yes, sound is a form of energy.

Question 24

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

Solution 24

Question 25

There is no atmosphere on moon. Can you hear each other on moon's surface? Can every kind of sound be heard?

Solution 25

There is no atmosphere on moon. So, we cannot hear each other on moon because sound needs a material medium to travel from one place to another. No kind of sound can be heard.

Question 26

State three characteristics of the medium required for the propagation of sound.

Solution 26

The following are the three characteristics of the medium required for the propagation of sound:
(i) The medium must possess elasticity so that the particles of medium have the tendency to return back to their original positions.
(ii) The medium must be frictionless so that there is no loss of energy during transmission.
(iii) The medium must possess inertia such that the particles of medium have the ability to store energy.

Question 27

Describe a simple experiment to determine the speed of sound in air.

Solution 27

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 stop watch. He stops the stop-watch as soon as he hears a echo. The distance travelled by the sound during the time (t) seconds is 2d.
So, speed of sound = distance travelled / time taken = 2d/t

Question 28

What are ultrasonic? State and explain two applications of ultrasounds.

Solution 28

Ultrasonics sound is the sound having frequency more than 20KHz. The application of ultrasonic sound is as follows:
(i) It is used to detect the flaws in metal casting of automobile tyres.
(ii) Used in hospitals to detect defects in certain parts of body.

Question 29

How are sound waves produced? How are sound waves transmitted from one place to the other?

Solution 29

Sound waves are produced by the object motion back and forth in the medium. Due to which it compresses the medium and compression waves are moved forward.  The sound travels in medium through energy transfers by wave motion from one place to another. For example , when the prong of the tuning fork vibrate , the waves start moving from one fork to another , the air start getting compressed . The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbor which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.

Question 30

Distinguish between light and sound waves. What are the consequences of the large difference in the speed of sound and that of light? Briefly explain them.

Solution 30

Light waves are electromagnetic waves while the sound waves are mechanical waves. Light waves do not require any material medium for their propagation while sound waves need the material medium for its travel from one place to another.
Due to large magnitude of difference between speed of light (i.e 3 X 10⁸ m/s) to the speed of sound in air (330 m/s) , the light reaches first and then the sound from the object. Like, in thunderstorm, the light reaches first to earth and then person hears the sound of light.

Question 31

How will the following factors affect the speed of sound in air:
(i) Pressure of air,
(ii) Temperature of air,
(iii) Humidity, and
(iv) Wind?

Solution 31

(i) Pressure of air : The speed of sound in air is independent of pressure of air.
(ii) Temperature of Air: Speed of sound in air is directly proportional to the temperature of air. As the temperature of air increases, the density of air decreases and hence speed of sound increases.
(iii) Humidity: The speed of sound increases with increase in humidity. The increase of moisture decreases the density of the atmosphere and therefore sound travels faster in moist air.
(iv) Wind: The speed of sound depends on the direction of wind. When the wind is blowing in the direction of propagation of sound, the speed of sound increases. But, when the wind blows in the direction opposite to that of the sound, the speed of sound decreases.

Question 32

Explain how the principle of echo is used by
(i) The bat during its flight at night, and
(ii) The dolphin to locate small fish as its prey.

Solution 32

(i) The bat during its flight at night ::: Bats emits its own distinctive sounds. The echoes of the sound helps bat to navigate and to locate their prey during night.
(ii) The dolphin to locate small fish as its prey ::: Dolphins sends the sound having frequencies between 120 KHz to 150KHz. The returning echoes used to help their preys.

Question 33

A person hearing at one end of a long metal bar hears two distinct sounds when the other end is struck with a stone. Explain, giving scientific reasons.

Solution 33

A person hearing at one end of the long metal bar hears two distinct sounds when the other end is struck with a stone because sound comes from the metal bar as well as from air. The sound travels faster in solids than air. So, first sound comes from metal bar hitting and then from air.

Question 34

Can sound travel through vacuum?

Solution 34

No, sound cannot travel through vacuum. It needs a material medium to travel.

Question 35

Does sound require a material medium for transmission?

Solution 35

Yes, sound requires a material medium for transmission.

Question 36

Name the factors on which the speed of sound depends.

Solution 36

The speed of sound depends on the following factors: Density of air, Temperature , Humidity in air as well the wind.

Question 37

What is the effect of wavelength and amplitude on the speed of sound?

Solution 37

There is no effect on the wavelength and amplitude on the speed of sound.

Question 38

What is the value of sound of room temperature?

Solution 38

Speed of sound at room temperature is 330 m/s

Question 39

Is the speed of sound different in different media?

Solution 39

Yes, the speed of sound is different in different medium.