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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