Oscillation

Oscillation, in physics, chemistry, and engineering, repeated motion back and forth past a central neutral position, or position of equilibrium. A single motion from one extreme position to the other and back, passing through the neutral position twice, is called a cycle. The number of cycles per second, or hertz (Hz), is known as the frequency of the oscillation.

A swinging pendulum or a plucked violin string eventually comes to rest if no further forces act upon it. The force that causes it to stop oscillating is called damping. Often the damping forces are frictional, but other damping forces, such as electrical or magnetic ones, might affect an oscillating system. See Electricity; Electronics; Magnetism.

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.

A dangerous type of vibration is the sudden, violent oscillating motion known as flutter. This occurs most frequently in the control surfaces of aircraft, but is also seen in the “galloping” of sleet-covered electrical transmission wires in high winds. One of the most spectacular instances of flutter caused the collapse in 1940 of the Tacoma Narrows Bridge in Tacoma, Washington. The collapse resulted from a gale that blew at a speed that enhanced the swaying of the bridge.

The amplitude of vibration of a structure might increase so rapidly when subject to flutter that the structure disintegrates almost instantaneously. Prevention of flutter is therefore an important consideration in the design of bridges and aircraft. Flutter analysis for aircraft is usually supplemented by tests on model aircraft carried out in a wind tunnel.

See also Harmonics; Oscillograph; Oscilloscope; Sound; Wave Motion.