Forced Vibrations occur when an object is forced to vibrate at a particular frequency by a periodic input of force.
Figure 4.23 shows a child playing on a swing while her friend tries to keep the oscillations going. An oscillations is any to-and-fro movement. We know from experience that the swing’s oscillation will eventually come to a halt aside from an external force acting on the swing. This is due to energy loss to the surrounding and this dissipation of energy is known as damping.
There are three kinds of damping as shown in Figure 4.24.