Selina Textbook Solutions: Propagation of Sound Waves | Physics Class 9 ICSE PDF Download

 Page 1


Chapter 8. Propagation of Sound Waves
Exercise 8(A)
Solution 1S.
Sound is caused due to vibrations of a body.
Solution 2S.
Sound is a form of energy that produces the sensation of hearing in our ears. Sound is 
produced by a vibrating body.
Solution 3S.
Vibrating
Solution 4S.
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.
Solution 5S.
Experiment to demonstrate that a material medium is necessary for the propagation of 
sound: 
Page 2


Chapter 8. Propagation of Sound Waves
Exercise 8(A)
Solution 1S.
Sound is caused due to vibrations of a body.
Solution 2S.
Sound is a form of energy that produces the sensation of hearing in our ears. Sound is 
produced by a vibrating body.
Solution 3S.
Vibrating
Solution 4S.
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.
Solution 5S.
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Keeping the key pressed, air is gradually withdrawn from jar by starting the vacuum 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.
Solution 6S.
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.
Solution 7S.
Requisites of the medium for propagation of sound:
1. The medium must be elastic.
2. The medium must have inertia.
3. The medium should be frictionless.
Solution 8S.
Page 3


Chapter 8. Propagation of Sound Waves
Exercise 8(A)
Solution 1S.
Sound is caused due to vibrations of a body.
Solution 2S.
Sound is a form of energy that produces the sensation of hearing in our ears. Sound is 
produced by a vibrating body.
Solution 3S.
Vibrating
Solution 4S.
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.
Solution 5S.
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.
Solution 6S.
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.
Solution 7S.
Requisites of the medium for propagation of sound:
1. The medium must be elastic.
2. The medium must have inertia.
3. The medium should be frictionless.
Solution 8S.
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. 
Solution 9S.
Page 4


Chapter 8. Propagation of Sound Waves
Exercise 8(A)
Solution 1S.
Sound is caused due to vibrations of a body.
Solution 2S.
Sound is a form of energy that produces the sensation of hearing in our ears. Sound is 
produced by a vibrating body.
Solution 3S.
Vibrating
Solution 4S.
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.
Solution 5S.
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.
Solution 6S.
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.
Solution 7S.
Requisites of the medium for propagation of sound:
1. The medium must be elastic.
2. The medium must have inertia.
3. The medium should be frictionless.
Solution 8S.
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. 
Solution 9S.
the disturbance
Solution 10S.
Sound travels in a medium in form of longitudinal and transverse waves.
Solution 11S.
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.
Solution 12S.
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.
Solution 13S.
A longitudinal wave propagates by means of compressions and rarefactions.
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.
Solution 14S.
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 
Page 5


Chapter 8. Propagation of Sound Waves
Exercise 8(A)
Solution 1S.
Sound is caused due to vibrations of a body.
Solution 2S.
Sound is a form of energy that produces the sensation of hearing in our ears. Sound is 
produced by a vibrating body.
Solution 3S.
Vibrating
Solution 4S.
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.
Solution 5S.
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.
Solution 6S.
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.
Solution 7S.
Requisites of the medium for propagation of sound:
1. The medium must be elastic.
2. The medium must have inertia.
3. The medium should be frictionless.
Solution 8S.
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. 
Solution 9S.
the disturbance
Solution 10S.
Sound travels in a medium in form of longitudinal and transverse waves.
Solution 11S.
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.
Solution 12S.
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.
Solution 13S.
A longitudinal wave propagates by means of compressions and rarefactions.
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.
Solution 14S.
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. 
Solution 15S.
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.
Solution 16S.
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.
Solution 17S.
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).
Solution 18S.
Solution 19S.
The distance travelled by a wave in one second is called its wave velocity. 
Its SI unit is metre per second (ms
-1
).