Question:

Why does only one of the pendulums reach large amplitude of oscillation?

Answer:

Each of the five pendulums have different lengths and hence a different natural
frequency. However, all of them were forced to oscillate at the driver’s
frequency. These are known as forced oscillations.

The amplitude attained by a forced oscillation depends on how closely the
driver frequency matches the resonant frequency. When the driver frequency
matches the resonant frequency exactly, the forced oscillation attains the
maximum possible amplitude. This is known as resonance.

As long as the damping is not severe, the resonant frequency of a pendulum
is very close to its natural frequency.

In this demonstration, the driver frequency is always adjusted (by adjusting
the length of the driver pendulum) to match the natural frequency of one of the
five pendulums. Hence only this pendulum goes into resonance while the other
four attain only very small amplitude.

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