Simple harmonic motion is a special type of periodic motion, in which a particle moves to and fro repeatedly about a mean position under a restoring force which is always directed towards the mean position and whose magnitude at any instant is directly proportional to the displacement of the particle from the mean position at that instant.

Restoring force µ Displacement of the particle from mean position.

                      F µ  – x

                      F = – kx

Where k is known as force constant. Its S.I. unit is Newton/meter and dimension is [MT ^–2].