The diagram above shows a wire stretched over a sonometer. Stems of two vibrating tuning forks A and B are touched to the wooden box of the sonometer. It is observed that the paper rider (a small piece of paper folded at the centre) present on the wire flies off when the stem of vibrating tuning fork B is touched to the wooden box but the paper just vibrates when the stem of vibrating tuning fork A is touched to the wooden box.
Why does the paper rider fly off when the stem of tuning fork B is touched to the box ?

Explanation

The paper rider flies off when the stem of the tuning fork B is touched to the box because the frequency of vibration of tuning fork B is equal to the natural frequency of vibration of the stretched wire holding the paper rider and resonance occurs.