In S.H.M., the displacement of the particle at an instant is given by:
y = rsin ωt.
Velocity, \(v=\frac{dy}{dt}\)= rω cos ωt
Acceleration, a =\(\frac{dy}{dt}\)= -ω2rsin ωt
= -ω2y.
From the above equation, we note that a ∝ y, i.e., acceleration in S.H.M. at an instant is directly proportional to the displacement of the particle from the mean position at that instant.