“Resonance is a special case of forced vibration”. This is because
All resonance vibrations are forced vibrations
When a body is subjected to forced vibration with a frequency which equal to or an integral multiple of the natural frequency of the body, the body vibrates with large amplitude violently. This vibration which is a special case of forced vibration is called resonance vibration & is due to complete transfer of energy from the body applying the force to the vibrating body.
Resonance occurs when two interconnected objects share the same vibrational frequency. Thus when one of the objects is vibrating, it forces the second object into vibrational motion and only if the frequency of the applied periodic force matches the natural frequency of vibration of the body there will be resonance. Thus resonance does not necessarily occur every time a body is subjected to forced vibration but it occurs when natural frequency of vibration of a body matches with that of the applied periodic force.