Forced Vibrations


Forced vibrations

Vibrations of a body under the constant influence of an external periodic force acting on it are called the forced vibrations. The external applied force is called the driving force. Amplitude of body oscillating under forced oscillations can be decreasing, constant or increasing depending on various factors like difference in amount of driving force and resistive force, difference in frequency of driving force and actual vibrations and difference in phase of driving force and actual vibrations.


Free vs forced vibrations

Free vibrationsForced vibrations
Vibrations of body in absence of any external force.Vibrations of body in presence of external driving force.
Frequency of vibrations depends on the source of vibrations.Frequency of vibrations depends on the source of vibrations and the frequency of the driving force.
Frequency of vibrations remains constant.Frequency of vibrations can be changed by changing frequency of driving force.
Amplitude of vibration is constant.Amplitude of vibration can be decreasing, increasing or constant.


Forced Oscillations

If an oscillator is displaced and then released it will begin to vibrate. If a force is continually or repeatedly applied to keep the oscillation going, it is a forced oscillation.