II.

Natural Frequency

Any oscillating object has a natural frequency, which is the frequency it tends to settle into if it is not disturbed. For example, the natural frequency of a pendulum 1 m (39 in) long is 0.5 Hz, which means the pendulum swings back and forth once every 2 seconds. If the pendulum is struck lightly once every 2 seconds, the amplitude of the swing increases gradually until the amplitude of oscillation is very large. The phenomenon in which a relatively small, repeatedly applied force causes the amplitude of an oscillating system to become very large is called resonance. Many of the serious vibration problems in engineering are caused by resonance. If, for example, the natural frequency of the body of a motor car is the same as that of the series of combustions in the engine when the car is travelling at a certain speed, the body might begin to vibrate or shake roughly. Such vibration can be avoided by mounting the engine on a damping material such as rubber to isolate the body from the engine.