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. See also resonance .