We consider a particle P moving along a circle of radius A with uniform angular velocity ω. Let N be the foot of the perpendicular drawn from the point P to the diameter XX’. Then N is called the projection of P on the diameter XX’. As P moves along the circle from X to Y, Y to X’, X’ to Y’ and Y’ to X. Thus as P revolves along the circumference of the circle, N moves to and fro about point O along the diameter XX’.The motion of N about O is said to be simple harmonic.Hence, Simple harmonic motion may be defined as the projection of uniform circular motion upon a diameter of a circle. The particle P is called reference particle or generating particle and the circle along which the particle P revolves is called circle of reference.
