In the case of forced vibration, if the frequency difference is small, the amplitude will be large (Figure). Ultimately when the two frequencies are same, the amplitude becomes maximum. This is a special case of forced vibration.
If the frequency of the external periodic force is equal to the natural frequency of oscillation of the system, then the amplitude of oscillation will be large and this is known as resonance.
(i) Using resonance, the frequency of a given tuning fork is determined with a sonometer.
(ii) In radio and television, using tank circuit, the required frequency can be obtained.
(i) Resonance can cause disaster in an earthquake if the natural frequency of the building matches the frequency of the periodic oscillations present in the Earth. The building begins to oscillate with large amplitude thus leading to a collapse.
(ii) A singer maintaining a note at a resonant frequency of a glass can cause it to shatter into pieces.