vibration, in physics, commonly an oscillatory motion—a movement first in one direction and then back again in the opposite direction. It is exhibited, for example, by a swinging pendulum, by the prongs of a tuning fork that has been struck, or by the string of a musical instrument that has been plucked. Random vibrations are exhibited by the molecules in matter (see Brownian movement). Any simple vibration is described by three factors: its amplitude, or size; its frequency, or rate of oscillation; and the phase, or timing of the oscillations relative to some fixed time (see harmonic motion). Sound is produced by the vibrations of a body and is transmitted through material media in pressure waves (see wave) made up of alternate condensations (forcing of the molecules of the medium together) and rarefactions (pulling of the molecules of the medium away from one another). In sound the vibration is longitudinal, for the movement is to and fro along the direction in which the sound is traveling. When a sound wave of one frequency strikes a body that will vibrate naturally at the same frequency, the vibration of the body is called sympathetic vibration. A reinforcement of sound resulting from sympathetic vibration is called resonance. When the vibrations of a sound-producing body cause another body to vibrate in the same frequency, not normally its own, the vibration is known as forced vibration. Heat is commonly defined as the energy of molecules, part of which consists of the energy of their vibrational motion.

In physics, the restraint of vibratory motion, such as mechanical oscillations, noise, and alternating electric currents, by dissipating energy. Unless a child keeps pumping a swing, the back-and-forth motion decreases; damping by the air's friction opposes the motion and removes energy from the system. Viscous damping is caused by such energy losses as occur in liquid lubrication between moving parts or in a liquid forced through a small opening by a piston, as in automobile shock absorbers. Hysteresis damping involves energy loss within the moving structure itself. Other types of damping include electrical resistance, radiation, and magnetic damping.

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Periodic back-and-forth motion (see periodic motion) of the particles of an elastic body or medium. It is usually a result of the displacement of a body from an equilibrium condition, followed by the body's response to the forces that tend to restore equilibrium. Free vibrations occur when a system is disturbed but immediately allowed to move without restraint, as when a weight suspended by a spring is pulled down and then released. Forced vibrations occur when a system is continuously driven by an external agency, as when a child's swing is pushed on each downswing. Because all systems are subject to friction, they are also subject to damping. In the example of free vibration, damping would cause the amplitudes of the spring's vibrations to diminish until eventually the system came to rest. Seealso resonance.

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