Simple Harmonic Motion:
Consider a particle moving round the circumference of a circle in an anticlockwise direction with a constant angular velocity as shown in below fig:
Let P be the position of the particle at any instant and N be the projection of P on the diameter XX’ of the circle. It will be noticed that when the point P moves round the circumference of the circle from X to Y, N moves from X to O. When P moves from Y to X’, N moves from O to X’. Similarly when P moves from X’ to Y’, N moves from X’ to O and finally when P moves from Y’ to X, N moves from O to X.
Hence, as P completes one revolution, the point N completes one vibration about the point O. This to and from motion of N is known as Simple Harmonic Motion (SHM).