ICSE Living Science Physics for Class 7 Solutions Chapter 6 Sound
Check Your Progress (Page 79)
A. Write true or false. Correct the false statements.
Question 1.
Sound is produced when a body vibrates.
Answer:
True.
Question 2.
The voice box responsible for the production of sound in human beings is located in the mouth.
Answer:
False. The voice box responsible for the production of sound in human beings is located in the throat.
Question 3.
Sound waves are transverse in nature.
Answer:
False. Sound waves are longitudinal in nature.
Question 4.
Frequency of a wave is the number of vibrations completed by it in one second.
Answer:
True.
Question 5.
Greater the amplitude of a vibration, greater will be the loudness of the sound produced.
Answer:
True.
B. Answer these questions.
Question 1.
Explain in brief about some sources of sound.
Answer:
Every vibrating body around us is a source of sound. A school bell, an alarm clock, a barking dog, moving vehicles and human voice are some sources of sound.
Question 2.
Why is a sound wave called a longitudinal wave?
Answer:
When a sound wave travels through a medium, a series of compressions and rarefactions is formed between the particles of the medium due to the vibration of these particles in a direction parallel to the direction in which the sound travels. This shows that sound waves are longitudinal in nature.
Question 3.
The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Answer:
Frequency = 100 Hz
So, number of vibrations completed by the sound in 1 second = 100
Number of vibrations completed by it in 1 minute = 60 x 100 = 6000
A. Tick the most appropriate answer
Question 1.
Sound is a form of energy which produces a sensation of
a. touch.
b. hearing.
c. sight.
d. feelings.
Answer:
b. hearing.
Question 2.
Sound is produced by alan
a. vibrating object.
b. object at rest.
c. rotating object.
d. rolling object
Answer:
a. vibrating object.
Question 3.
Which of the following is transferred by a wave from one place to another?
a. mass
b. particles
c. energy
d. matter
Answer:
c. energy
Question 4.
If five waves are produced in one second, then the time period of the wave is
a. 0.2 s.
b. 0.25 s.
c. 0.1 s.
d. 0.05 s.
Answer:
a. 0.2 s.
Question 5.
Sound cannot propagate through
a. solids.
b. liquids,
c. gases.
d. vacuum.
Answer:
d. vacuum
Question 6.
Which of the following is a good sound absorber ?
a. glass
b. water
c. thermocol
d. vacuum
Answer:
c. thermocol
Question 7.
Which of the following concepts is used in SONAR to find the depth of oceans ?
a. musical sound
b. echoes
c. laser
d. noise
Answer:
b. echoes
Question 8.
The speed of sound in air at 0 °C is approximately
a. 344 m/s.
b. 260 m/s.
c. 330 m/s.
d. 390 m/s.
Answer:
c. 330 m/s.
B. Fill in the blanks.
Question 1.
An object that does not vibrate cannot produce ………….
Answer:
An object that does not vibrate cannot produce sound.
Question 2.
Sound travels in the form of …………
Answer:
Sound travels in the form of waves.
Question 3.
Vibration means …………. motion of an object
Answer:
Vibration means to-and-fro motion of an object.
Question 4.
The larynx has two folds called ………….
Answer:
The larynx has two folds called vocal cords.
Question 5.
Sound waves are in nature.
Answer:
Sound waves are longitudinal …………. in nature.
Question 6.
The SI unit of frequency is ………….
Answer:
The SI unit of frequency is hertz.
Question 7.
Loudness of a sound depends upon the …………. of the sound.
Answer:
Loudness of a sound depends upon the amplitude of the sound.
Question 8.
The repetition of sound produced due to the reflection of sound waves from a reflecting surface is known as an
Answer:
The repetition of sound produced due to the reflection of sound waves from a reflecting surface is known as an echo.
C. Write true or false. Correct the false statements.
Question 1.
Sound is produced by vibrating bodies.
Answer:
True.
Question 2.
Loudness of a sound depends upon the frequency of the sound.
Answer:
False. Loudness of a sound does not depend upon the frequency of the sound.
Question 3.
Sound travels fastest in vacuum.
Answer:
False. Sound travels fastest in solid.
Question 4.
The human ear can hear two sounds separately only if they reach the ear after an interval of one-twentieth of a second.
Answer:
False. The human ear can hear two sounds separately only if they reach the ear after an interval of one-tenth of a second.
Question 5.
SONAR is used to produce and transmit low frequency sound waves.
Answer:
False. SONAR is used to produce and transmit high frequency sound waves.
D. Answer the following in short.
Question 1.
What do you understand by the term wave?
Answer:
A wave is a disturbance produced in a medium that carries energy from one part of the medium to another part of the medium, without the transfer of matter.
Question 2.
Differentiate between transverse and longitudinal waves.
Answer:
A wave in which the particles of the medium vibrate up and down, that is, perpendicular to the direction in which the wave . is moving, is called a transverse wave. On the other hand, a wave in which the particles of the medium vibrate to-and-fro in the same direction in which the wave is moving, is called ” a longitudinal wave.
Question 3.
How are loudness and amplitude related?
Answer:
Loudness of a sound depends on its amplitude. Higher the amplitude, louder is the sound. As the amplitude of a wave is directly related to the energy it carries, we can say that greater is the energy carried by a wave, louder is the sound produced.
Question 4.
What is the time period of a particle that is vibrating at a frequency of 10 Hz?
Answer:
Frequency,f = 10 Hz
So, 10 vibrations are completed in 1 s.
Therefore, time period of the sound wave = \(\frac {1}{10}\) s = 0.01 s
Question 5.
Why are the walls and floors of cinema halls and auditoriums covered with curtains and carpets?
Answer:
The walls, ceilings and floors of auditoriums and cinema halls are-covered with sound-absorbing materials such as thick sound-proofing tiles, curtains and carpets. These materials help reduce reverberations.
Question 6.
Why do echoes produced in an empty auditorium usually decrease when it is full of people ?
Answer:
People absorb the sound waves travelling in the air and reduce reverberations. Whereas, in an empty auditorium, all sound waves get reflected by the walls of the autitorium.
Question 7.
Describe elasticity of a medium.
Answer:
Elasticity is the property of a medium to regain its shape after being stretched or compressed.
E. Answer the following in detail.
Question 1.
Describe sound as a longitudinal wave.
Answer:
When a sound wave travels through a medium, a series of compressions and rarefactions is formed between the particles of the medium due to the vibration of these particles in a direction parallel to the direction in which the sound travels. This shows that sound waves are longitudinal in nature.
Question 2.
Define amplitude, time period and frequency with respect to a sound wave. State the relationship between the time period and the frequency of a wave.
Answer:
Amplitude is the maximum displacement of a wave on either side of its mean position. Time period of a wave is the time taken by it to complete one full oscillation. It is denoted by T and is measured in seconds. Frequency of a sound wave is the number of oscillations completed by it in one second. It is denoted by/and its unit is hertz (Hz).
Question 3.
What does’ it mean if we say that a tuning fork has a frequency of 256 Hz?
Answer:
If a tuning fork has a frequency 256 Hz, then it means that it vibrates 256 times in 1 second.
Question 4.
How can you prove experimentally that sound cannot propagate through vacuum ?
Answer:
Aim : To show that sound cannot propagate through vacuum.
Materials Required : An electric bell, a glass bell jar, vacuum pump, battery and a swtich.
Procedure :
Step 1. : Place the electric bell in the glass bell jar, as shown in fig. 3.4. Make sure that all the openings of the bell jar are properly closed. Now complete the electric circuit by switching on the switch.
Observation : When the circuit is closed, you can hear the sound of the bell. Also you can see its hammer striking against the gong.
Step 2.
Now, slowly remove the air from the bell jar by using the vacuum pump connected to the jar. Can you hear the bell now ?
Observation : As air is removed from the bell jar, the loudness of the bell decreases until it becomes very faint. Finally, you will not be able to hear the bell even though you can see the hammer of the bellstriking the gong. If you allow air to enter the bell jar, you will hear the sound of the bell again.
Conclusion : Sound cannot propagate in the absence of a material medium, that is, sound cannot travel through vacuum.
Question 5.
What is an echo ? What are the necessary conditions for its occurrence?
Answer;
An echo is defined as a repetition of sound produced by the reflection of sound waves from a reflecting surface.
Conditions required for occurrence of an echo are :
- The minimum distance between the source of sound and the reflecting surface is around 17 m.
- There is a high wall or a hill or a high-rise building, which can act as a reflecting surface.
- The sound is loud enough so that it reaches the receiver after reflection.
Question 6.
What is SONAR? How is it used to determine the depth of an ocean ?
Answer:
Sound Navigation And Ranging (SONAR) is an instrument that uses the concept of echo (or reflection of sound) to measure the depth of oceans. This instrument produces and transmits high frequency (ultrasonic) waves towards the seabed, from any point, say A on a ship/boat. After striking the seabed, these waves are reflected upward and received at another point, say B. The time elapsed between transmitting and receiving back these waves is recorded. Using the speed of sound in sea water as 1560 m/s at 20 °C, and the elapsed time, the depth of the seabed can easily be calculated.
Question 7.
List the factors on which the speed of sound depend ?
Answer:
The speed of sound in a medium depends mainly upon the elasticity and the density of the medium. The speed of sound increases as the density of the medium increases. Since the density of solids is the maximum, followed by liquids and gases, sound travels fastest through solids, little slower through liquids and slowest through gases or air. The speed of sound also depends upon the pressure and speed of the fluid medium, temperature and humidity.
F. Solve the following numerical problems.
Question 1.
What is the time period of a wave with a frequency of 250 Hz?
Answer:
Given, Frequency = 250 Hz
T = \(\frac {1}{f}\) = \(\frac {1}{250}\)= 0.004 second
Question 2.
What is the frequency of a wave whose time period is 0. 015 seconds ?
Answer:
Time period = 0.015 seconds
Frequency = ?
We know that,
= 66.66 Hz
Question 3.
Calculate the minimum distance required for an echo to be heard.
Answer:
We know that, speed of the sound in air is 344 m/s
and minimum time required to hear an echo = \(\frac {1}{10}\) s = 0.1 s
We know,
distance = speed x time = 344 x 0.1 = 34.4 m
As the sound travels twice between the source and the reflecting surface before an echo is heard, the minimum distance required to hear an echo is \(\frac {34.4}{2}\) m = 17.2 m
Question 4.
A boy fires a gun and hears the echo 2 seconds later. If he is 350 m away from a wall, calculate the velocity of sound in air.
Answer:
Distance travelled by sound = 350 x 2 = 700 m.
Time taken = 2 s
Question 5.
A girl claps and hears the echo after reflection from a cliff which is 660 m away from her. If the velocity of sound is 330 m/s, calculate the time taken for hearing the echo.
Answer:
Distance travelled by sound = 660 x 2 = 1320 m
Speed of sound = 330 m/s
G. Look at the figure given below and answer the following questions.
Question 1.
Identify the regions A and B.
Answer:
Region A shows compression while Region B shows rarefaction.
Question 2.
Write the difference between the regions A and B.
Answer:
When a vibrating body moves outwards, it increases pressure on the molecules of the medium surrounding it, for example, air and causes them to come close to each other. This is known as compression. When the vibrating body moves inwards, the pressure on the molecules gets reduced. So, the gaps between the molecules increase causing them to spread out. This is known as rarefaction.
Question 3.
Which type of wave motion is represented in this figure?
Answer:
Longitudinal wave motion is represented in the given figure.
H. Look at these figures and answer the following questions.
Question 1.
In which case the loudness of the sound is greater and why?
Answer:
The sound wave shown in Figure 1 is louder as it has higher amplitude.
Question 2.
Name the factor based on which you drew the above conclusion.
Answer:
The factor on which we drew the above conclusion is the amplitude. Loudness of a sound depends on its amplitude. Higher the amplitude, louder is the sound.
Question 3.
Write some other factors on which the loudness of a sound depends.
Answer:
Some other factors on which the loudness of a sound depends are the atmospheric temperature, pressure, velocity of the wind and the amount of moisture present in the air.
Think And Answer
Question 1.
Why cannot two friends talk to each other if they are on the moon?
Answer:
Sound requires a medium for propagation. As there is vacuum on the moon, two friends cannot hear each other there.
Question 2.
Why sound travels faster on a hot summer day than on a cold winter day?
Answer:
As speed of the sound is directly proportional to the temperature of the medium through which it propagates, therefore sound travels faster on a hot summer day than on a cold winter day.
Question 3.
A person presses his ears against the railway track to find whether a train is approaching or not. Why?
Answer:
The speed of sound increases as the density of the medium increases. Since the density of solids is the maximum, followed by liquids and gases, sound travels fastest through solids, little slower through liquids and slowest through gases or air. This is the reason why a person presses his ears against the railway track to find whether a train is approaching or not.
Living Science Physics Class 7 ICSE Solutions