April 3, 2020

# Important Questions Answer of Physics – II | Wave Motion | Grade XII

## Wave Motion

1. Differentiate between forced vibration and free vibration.

Ans: The free vibration and forced vibration can be differentiated by the following points:

i. A body is said to execute free vibration when it is vibrating by itself, while a body is said to execute forced vibration , when it is compelled to vibrate by a periodic force at a frequency other than its natural frequency.

ii. A body in free vibration vibrates with its natural frequency while in forced vibration it vibrates with the frequency of the source.

iii. A body in free vibration vibrates with the help of internal elastic forces while in forced vibration it vibrates with the help of external periodic force.

iv. Free vibration is self-sustained vibration while forced vibration is externally sustained vibration.

2. How are stationary waves formed?

Ans: When two progressive waves of the same wavelength and amplitude travel with the same speed through a medium in opposite directions, they superimpose upon each other and thus a wave is formed which is stationary wave.

When a stationary wave is formed, points of maximum and minimum, (zero) amplitudes are resulted alternately in space, which is shown in the figure.

where A is antinode at which there is maximum displacement and N is node where there is zero displacement.

3. If you are walking on the moon surface, can you hear the cracking sound behind you? Explain.

Ans: No, we can’t hear the cracking sound behind us, if we are walking on the moon surface. This is so because there is no atmosphere on the moon. So there is lack of medium for the propagation of sound waves on the moon. As we know that sound waves need medium for propagation, it cannot propagate in vacuum.

4. Do sound waves undergo reflection, refraction and polarization phenomena? Explain.

Ans: Being a wave, the sound wave undergoes reflection and refraction. These phenomena are exhibited by both transverse wave and longitudinal wave. But, polarization phenomenon is exhibited by transverse wave only. Since the sound wave is longitudinal wave, so it doesn’t exhibit polarization phenomenon.

5. Which types of wave propagate in liquid, explain.

Ans: Both transverse wave and longitudinal wave are possible in liquid. Between these two waves, major component of the wave is longitudinal which dominates transverse wave. A liquid can have bulk modulus of elasticity but it doesn’t have modulus of rigidity.

6. Distinguish between light waves and sound waves.

Ans: The following points distinguish between light waves and sound waves.

7. A radio station broadcasts at 800 KHz. What will be the wavelength of the wave?

Ans: Given, frequency of sound wave (f) = 800 KHz = 800000 Hz

Also, we know velocity of sound in air (v) = 32 m/s

Then, the wavelength of the wave is given by

λ=v⁄f=332/800000=4.15×10¯4m

Hence, the wavelength of the wave will be 4.15×10¯4m if a radio station broadcasts at 800 KHz.