ICSE-IX-Physics
08: Propagation of Sound Waves
Note: Please signup/signin free to get personalized experience.
Note: Please signup/signin free to get personalized experience.
10 minutes can boost your percentage by 10%
Note: Please signup/signin free to get personalized experience.
- #
- #Section : A
- Qstn #2What is sound? How is it produced?Ans : Sound is a form of energy that produces the sensation of hearing in our ears. Sound is produced by a vibrating body.
- Qstn #4Describe a simple experiment which demonstrates that the sound produced by a tuning fork is due to vibrations of its arms.Ans : 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.
- Qstn #5Describe in brief, with the aid of a sketch diagram, an experiment to demonstrate that a material medium is necessary for propagation of sound.Ans : 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Keeping the key pressed, air is gradually withdrawn from jar by starting the vacuum pump.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.
- Qstn #6There is no atmosphere on moon. Can you hear each other on the moon’s surface?Ans : 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.
- Qstn #7State three characteristics of the medium required for propagation of sound?Ans : Requisites of the medium for propagation of sound:
- The medium must be elastic.
- The medium must have inertia.
- The medium should be friction less.
- Qstn #8Explain with an example, the propagation of sound in a medium.Ans :
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
(d), 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.
- Qstn #9Choose 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.Ans : When sound travels in a medium the disturbance travels in form of a wave.
- Qstn #10Name the two kinds of waves in form of which sound travels in a medium.Ans : Sound travels in a medium in form of longitudinal and transverse waves.
- Qstn #11What is a longitudinal wave? In which medium: solid, liquid or gas, can it be produced?Ans : 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.
- Qstn #12What is a transverse wave? In which medium: solid, liquid or gas, can it be produced?Ans : 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.
- Qstn #13Explain meaning of the terms compression and rarefaction in relation to a longitudinal wave.Ans : 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.