Resonant Pendulums

Here we have six pendulums consisting of three pairs of equal lengths. All are suspended from the same supporting rod, which is free to move back and forth. Now, set one of the pendulums into motion. You will notice that another pendulum whose length is the same starts oscillating with maximum displacement.

This is because of the maximum transfer of energy of motion of one pendulum to the other of same length. This is the principle of resonance. This principle can be used in the resonant oscillations of two swings of same length.