When the period of the force is not the same as the free
period of the body, the body ultimately vibrates in time with
the force, and its vibrations are called Forced Vibrations. In
the special case where the period of the force and the free
period of the body are the same, we have the phenomenon
known as Resonance. As the latter case is the simpler, we
shall take it first.
158. Resonant Vibrations of a Pendulum.
Make a simple pendulum by suspending a heavy bob by a
string of such a length that the centre of the bob is 39 inches
below the point of suspension. This pendulum will have a
period of about two seconds, that is, at intervals of two
seconds it will be found in a particular phase, say at the end
of its swing to the right. Fix two light threads to the bob.
