Ans. A simple pendulum consists of a small bob of mass 'm' suspended from a light string of length L fixed at its upper end with a support. When the bob is at the mean position O, the net force acting on the bob is zero and the bob is stationary. Now if we bring the bob to the extreme position A, the net force acting on the bob is not zero. As the component of the weight of the bob mass G is balanced by the tension in the string, therefore, there is no force acting along the string.
Hence, there is no motion along this direction. The component of weight mass G is directed towards the mean position and acts as a restoring force. Due to this force, when the bob is released from point A, it starts moving towards the mean position O. At the mean position O, the velocity of the bob is maximum and due to inertia, the bob does not stop at point O rather it continues to move towards the extreme position B. The velocity of the bob becomes zero on reaching at point B.
On reaching at point B, the bob starts moving
towards the mean position O due to the restoring force mg sin 6. In this way, the bob continues its to and fro motion between the points A and B about the mean position O.
It is clear from the above discussion that acceleration of the bob is always directed towards the mean position O. Hence the motion of the simple pendulum is SHM.