Demonstrate resonance and forced vibrations
Consider four simple pendulums P,Q,R and S suspended from horizontal stretched rubber card AB. Let the lengths of pendulums p and R be equal Q is shorter and S is longer than P. If the pendulum P is pulled aside perpendicular to AB and released, after a short time, the pendulum R starts vibrating with increasing amplitude while Q and S remain at rest. The length of pendulums P and R being equal, their frequencies are equal. Hence they are in resonance.
The length of P,Q,S are different. Their amplitudes are also different. Hence Q and S are said to be in a state of forced vibrations since they differ in frequencies among themselves and also with P.