PHYSICAL CHARACTERISTICS OF FORCED HARMONIC MOTION

We can observe the main physical characteristics of forced harmonic motion using a simple pendulum. We drive the pendulum by moving its point of suspension backwards and forwards harmonically, along a horizontal direction. At very low driving frequencies the pendulum mass closely follows the movement of the point of suspension with them both moving in the same direction as each other, i.e. they have the same amplitude and move in phase. As the driving frequency is increased the amplitude of oscillation increases dramatically and becomes much larger than the movement of the point of suspension. We might rightly suspect that the maximum amplitude occurs when the pendulum is driven close to its natural frequency of oscillation. The system is then said to be in resonance. We get the largest amplitude at resonance because this is the frequency at which the pendulum ‘wants’ to oscillate. As the driving frequency is increased further the amplitude of oscillation decreases but perhaps more surprisingly the mass now moves in the opposite direction to the point of suspension, although still with the same frequency.
At even higher frequencies we reach the situation where the pendulum mass hardly moves at all. This is because it has inertia. The simple pendulum serves as a useful example, but all forced oscillators behave in this manner.