Damped Oscillation :
If there is only restoring force acting on a vibrating object for example simple pendulum or tuning fork then that object can vibrate till infinity. This type of vibration or oscillation is called ‘free oscillation’ and this type of vibration is called ‘free vibration’. Its frequency is called ‘Natural Frequency’. There is only restoring force working in free vibration. Its amplitude and energy both are constant with time as shown in figure.
Fig: Free oscillations of equal amplitude
Forced Vibrations :
We have noticed that when the motor of flour mill, or motor of well is started then the objects closer to it like windows, doors also vibrate. This is called forced vibrations.
According to fig. if four simple pendulums A, B, C and D are hanged and out of these pendulum A is vibrated; then we will notice that B, C and D also vibrate with the frequency of A Vibrations of B, C and D are called forced vibrations.
Fig: Forced vibrations
Resonance :
In forced oscillations when the frequency of the external periodic forcé exerted on an oscillator is equal to the natural frequency of the oscillator then in this condition the amplitude of the forced oscillations becomes very high. This special state of forced oscillations is called resonance. In this condition maximum energy is transferred from the driver to the driven due to which the amplitude of oscillations of the driven increases greatly.
Examples of Resonance:
(i) When a vibrating tuning fork is placed in a hollow box which contains air, then when the natural frequency of air is equal to the tuning fork’s frequency, a very intense sound is heard.
(ii) While crossing a bridge a group of soldiers do not put their foots together.
(iii) During earthquake the natural frequency of the buildings becomes equal to the frequency of the Earth’s vibrations Hence they start vibrating and collapse.
(iv) We hear the radio broadcasting programs by tuning our radio transistors.
(v) While crossing a bridge or tunnel the driver does not blow the whistle of the train.
