A small ball is held in place by a stretched string, one end fixed to a wall and the other attached to a vibration generator. The natural frequency of the ball's oscillations is that found from its free oscillations (about 3.7 Hz). The vibration generator is switched on and its frequency f of vibration is gradually increased from 0 to 10 Hz.

On Fig. 4.3, sketch the variation with f of the amplitude of the oscillations of the ball.