Differentiate between forced vibration and free vibration? [HSEB 2052]

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

1) 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.
2) A body in free vibration vibrates with its natural frequency while in forced vibration it vibrates with the frequency of the source.
3) 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.
4) Force vibration is self-sustained vibration while forced vibration is externally sustained vibration.

What do you mean by reversible adiabatic process? [HSEB 2051]

An expansion or contraction of a gas in which no heat enters or leaves the gas is called an adiabatic expansion or contraction. A reversible adiabatic change is an adiabatic change which can be retraced in the opposite direction so that it passes through exactly the same states in all respects as in the direct process. It requires conditions such as a light frictionless piston, very slow expansion, a constant temperature reservoir with very thin good conducting walls etc. It is an ideal case.

Solid can propagate both transverse and longitudinal wave whereas liquid and gases can propagate only longitudinal wave, Why?

The rigidity modulus and bulk modulus of medium are responsible for propagation of transverse and longitudinal waves respectively. Solid has both rigidity and bulk modulus, it can propagate both waves. But liquid and gases have only bulk modulus poor rigidity modulus. So they can propagate only longitudinal wave.