Forced oscillations. Resonance in technology
In a real medium free oscillations are damped by friction: the amplitude decreases and the energy turns into heat. To keep the oscillations from dying out, a periodically varying external – driving – force must act on the system; such oscillations are called forced oscillations. The frequency of forced oscillations equals the frequency of the driving force. When the driving frequency ν approaches the natural frequency ν₀ of the system, the amplitude increases sharply; at ν = ν₀ it is greatest – this is resonance. With little friction the resonance peak is higher and narrower. Resonance is used when pushing a swing at the right moments, tuning a radio receiver to a wave, and in musical instruments. It is harmful when it destroys bridges, buildings or machine parts; so soldiers cross a bridge out of step, and machines are built with vibration damping.
On the school playground, only under the teacher’s supervision: push a swing gently at different rhythms and observe at which rhythm it rises most. The child on the swing must sit properly; do not swing it very high and do not stand in front of it.