Any motion which changes it after regular interval of time is known as periodic or harmonic motion. When the body of a pendulum moves to and fro through a short angle, the bob is known as move simple harmonically. Current can be considered simple harmonic because of simple harmonic vibrations of electric and magnetic forces. In part of periodic motion, force is always directed towards a fixed point which may or may not be on the way of motion. Simple harmonic motion is closely linked with circular rotation. An example is shown below. This shows an arrangement used to convert circular motion of a stone D into the to and fro or simple harmonic motion of a point P.