Definitions, equations and graphs
Definition of Simple Harmonic Motion (SHM): LO (d)
A motion where the acceleration a of an object is always directed towards a fixed point ( the equilibrium position O ) and proportional to its displacement x from the fixed point is said to be be simple harmonic.
Defining equation of SHM
a = - ω2 x
Observe from the model or the animation, suggest the reason for the negative sign.
The negative sign means a is always directed towards the equilibrium point while x is pointing away from equilibrium point O.
In other words a and x are in the opposite direction.