Simple harmonic motion is any motion in which the displacement is reversed by an opposing restoring force that is proportional to the displacement. Or, to put it another way, the more you pull it, the more it wants to turn around and head back in the direction of the centre. An illustration of this is the spring-block oscillator, which exerts the greater force back toward the equilibrium position the more you stretch it. The acceleration of the particle at any position is directly proportional to the displacement from the mean position, and it can be thought of as an oscillatory motion. This particular oscillatory motion is a special case.