# Important Questions and Answers: Wave Motion

1. How are stationary waves formed?

Ans: Stationary (standing) waves are those which cannot carry energy from one end to the other end. Their formation is possible when two waves of same frequency travelling with same velocity in opposite directions interact with each other. This makes the resultant velocity of the waves zero. However, since energy is flowing in both directions; it gets locked in between the nodes. This makes their amplitude increase to very high values.

2. Do sound waves need a medium to travel from one point to other point in space? What properties of the medium are relevant?

Ans: Of course, a medium is necessary for the propagation of a sound wave. Since sound is longitudinal wave, the medium must meet two requirements. First the medium should have elasticity so that particles can return to equilibrium or mean position after having been disturbed. The medium must have inertia so that particles are able to maintain their previous state of rest or motion. (This is useful when particles move towards the mean position. Their inertia helps them in going beyond the mean position. )

3. What types of waves propagate in a liquid? Explain.

Ans: Liquid molecules are mobile and also can easily slide (shear) over one another. Besides they are highly incompressible. So, inside the liquid, the waves are longitudinal, the velocity being higher that in air. At the surface however, the waves are transverse. So, both transverse and longitudinal wave can propagate in liquid.

4. Can we talk to each other on the surface of moon?  or can you hear the cracking sound behind you?

Ans: The direct answer is “No”. It is impossible to talk to each other and no sound is heard. Sound propagates only through the material medium. There is no atmosphere on the moon. Therefore sound wave cannot travel there.

5. Can transverse waves be produced in air ?

Ans: No, for a transverse wave to propagate, the body should possess modulus of elasticity. Since air has no modulus of elasticity, transverse wave cannot be produced in air.

6. When a stone is thrown on the surface of the water, the waves travel out. From where does this energy come from?

Ans: When a stone is thrown, its potential energy get converted into kinetic energy. As soon as the stone strikes with the surface of water, its velocity decreases and the energy is transferred to water molecules thereby forming ripples.

7. Differentiate between transverse wave and longitudinal wave.

 Transverse wave Longitudinal wave The wave in which the particles of the medium vibrate mutually perpendicular to the direction of propagation is called transverse wave. The wave in which the particles of the medium vibrate along the direction of propagation of wave is called longitudinal wave. It travels by making a series of crest and trough. It travels by making a series of compression and rarefaction. It can propagate in solid and the surface of the liquid. It can propagate in all type of medium: solid liquid and gas. The pressure remains constant throughout all points. The pressure becomes maximum at compression and minimum at rarefaction.

8. What do you mean by frequency and wavelength of the wave?

Ans: Frequency is the total number of oscillation made by the vibrating particle in one second. It Is denoted by F.

Wavelength is the distance between two successive crest and trough or compression or rarefaction.

9. Differentiate between progressive waves and stationary waves.

 Progressive wave Stationary wave The wave in which there is a transfer of energy and momentum from one particle to another particle is called progressive wave. The wave in which there is no transfer of energy and momentum between the particles of the wave is called stationary wave. No particles are permanently at rest. The particles at nodes are permanently at rest. There is a variation of pressure at every point. Pressure variation is maximum at nodes and minimum at antinode. The amplitude of oscillation is same at every point in the medium. The amplitude of oscillation is zero at node and maximum at antinode.

10. When water is poured into an extremely hot vessel, it gives a sizzling sound, why?

Ans: Such phenomenon is more related to the sudden high pressure created in the water-vessel boundary. It is to be noted that sound is produced due to compression and rarefaction produced in the producing substance and then in the medium, which requires that there be some pressure difference. When water comes in contact with hot vessel, water suddenly reaches boiling point there and turn to vapour. It tries to expand suddenly causing a large pressure. This pressure pushes neighboring atoms and suddenly starts a series of compression and rarefaction which then give the impression of sound.

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#### 6 Responses to “Important Questions and Answers: Wave Motion”

1. ngonji dizzy

2. ngonji dizzy

3. Maryam Aziz

This is really good question and answers. Thank you for typing.

4. Excel

I need good question now

5. ganesh kumarsharma

very nice question(thanku)

6. yusra nihar

the speed of sound in solid is greater than in air