To understand resonance,describe the experiment of oscillations of five pendulums.

(N/A) Consider a set of five simple pendulums of assorted lengths suspended from a common horizontal rope,as shown in the figure.
Pendulums-$1$ and $4$ have the same lengths,while the others have different lengths. Let us set pendulum-$1$ into motion. The energy from this pendulum is transferred to the other pendulums through the connecting rope,causing them to start oscillating.
Pendulums-$2$,$3$,and $5$ initially start oscillating with their own natural frequencies and different amplitudes,but this motion is gradually damped and not sustained. Their frequencies of oscillation gradually change,and ultimately,they oscillate with the frequency of pendulum-$1$ (the driving frequency),but with different amplitudes.
Pendulum-$4$ behaves differently from the others. Since it has the same length as pendulum-$1$,it has the same natural frequency. It oscillates with the same frequency as pendulum-$1$,and its amplitude gradually increases,becoming very large. This is the phenomenon of resonance.
In general,a system may have several natural frequencies,for example,vibrating strings,air columns,etc.