Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest ?

Explanation

Initially, the simple pendulum is at rest with its bob in the centre position (or mean position) A. When the pendulum bob is pulled to one side to position B (to give it potential energy because of higher position of B with respect to position A), and then released, the bob starts swinging (moving back and forth) between position B and C (see Figure).

(i) When the pendulum bob is at position B, it has only potential energy (but no kinetic energy).
(ii) As the bob starts moving down from position B to position A, its potential energy goes on decreasing but its kinetic energy goes on increasing.
(iii) When the bob reaches the centre position A, it has only kinetic energy (but no potential energy).
(iv) As the bob goes from position A towards position C its kinetic energy goes on decreasing but its potential energy goes on increasing.
(v) On reaching the extreme position C, the bob stops for a very small instant of time. So at position C, the bob has only potential energy (but no kinetic energy).
From the above discussion we conclude that at the extreme positions B and C of a swinging pendulum, all the energy of pendulum bob is potential, and at the centre position A, all the energy of the pendulum bob is kinetic. At all other intermediate positions, the energy of pendulum bob is partly potential and partly kinetic. But the total energy of the swinging pendulum at any instant of time ramains the same (or conserved).
The swinging pendulum bob eventually comes to rest because it loses energy due to the friction at the point of support of the pendulum and friction of air (or air resistance) acting on the swinging pendulum bob. This energy is converted into heat energy and sound energy which go into the environment.