When a bob of a simple pendulum (in
vacuum) is displaced from its mean position and left, it executes simple
harmonic motions. The pendulum will continue to oscillate with the same time
period and amplitude for any length of time. In such cases there is no loss of
energy by friction or otherwise. In all similar cases, the vibrations will be
undamped free vibrations.
Damped free vibrations:
In actual practice,
when the pendulum vibrates in air medium, there are frictional forces and
consequently energy disappear in each vibration. The amplitude of swing
decreases continuously with time and finally the oscillations die out. Such
vibrations are called damped free vibrations.
The time period of a body executing simple
harmonic motion depends on the dimensions of the body and its elastic
properties. The vibrations of a body die out with time due to loss of energy.
If some external periodic force is constantly applied on the body, it continues
to oscillate under the influence of such external forces. Such vibrations of
the body are called forced vibrations.