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Question
Describe a simple experiment to illustrate the phenomenon of resonance and explain it.
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Solution
Four pendulums A, B, C, and D are suspended from a rubber string PQ. The pendulums A and B are of equal lengths while C is shorter and D is longer. The pendulum A is set into vibration. It is observed that the pendulum B also starts vibrating and ultimately acquires the same amplitude as that of A. The pendulums C and D also vibrate, but with smaller amplitudes. The pendulum B is said to be in resonance with pendulum A.
Explanation: The natural frequency of pendulum B is equal to the frequency of vibration of pendulum A since both are of equal lengths. So the forced vibrations caused in pendulum B are in resonance with vibrations of pendulum A. On the other hand, the natural frequency of pendulum C is higher while of D is smaller than that of pendulum A. So these pendulums vibrate with the frequency of pendulum A with smaller amplitudes due to forced vibrations.
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