Q.

The periodic motion in which a particle moves to and fro about a fixed point such that its acceleration is directly proportional to its displacement but in opposite direction and acceleration always acts towards its mean position then such motion is called simple harmonic motion

 If “a” is acceleration and y is displacement of a particle then $\mathrm{a}\propto -\mathrm{y}$ (or) a = -ky here k is proportional constant .

-ve sign indicate that “a” and y are in opposite direction.

Ex: 1) Oscillation of a loaded spring
      2) Vibration of the prongs of tuning fork
      3) The projection of uniform circular motion
           on any diameter
      4) Oscillation of simple pendulum