Forced vibration of S.D.O.F system
Forced vibration is the one in which external energy is added to the vibrating system. The amplitude of a forced-undamped vibration would increase over time until the mechanism was destroyed. The amplitude of a forced-damped vibration will settle to some value where the energy loss per cycle is exactly balanced by the energy gained.
Examples of this type of vibration include a structure subjected to vibrating machines, vibration of a building during an earthquake or under the action of wind, etc.