"A vibration is the periodic motion of a body or system of connected bodies displaced from the position of equilibrium. In general, there are two types of vibration, free and forced. Free vibration occurs when the motion is maintained by gravitational or elastic restoring forces, such as the swinging motion of a pendulum or the vibration of an elastic rob. Forced vibration is caused by an external periodic or intermittent foirce applied to the system. Both of these types of vibration may be either damped or undamped. Undamped vibrations can continue indefinitely because frictional effects are neglected in the analysis. Since in reality both internal and external frictional forces are present, the motion of all vibrating bodies is actually damped." p595