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.

Question.
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 ? What happens to its energy eventually ? Is it a violation of the law of conservation of energy ?

Solution:

When the pendulum bob is pulled (say towards left), the energy supplied is stored in it in the form of potential energy (PE) on account of its higher position. When the pendulum is released so that it starts moving towards right, then its PE changes into kinetic energy (KE), such that in mean position, it has maximum KE, and zero PE. As the pendulum moves towards extreme right, its KE changes into PE; such that at the extreme position, it has maximum PE and zero KE. When it moves from this extreme position to mean position, its PE again changes to KE. This means total mechanical energy remains constant. This illustrates the law of conservation of energy. Eventually, the bob comes to rest, because during each oscillation a part of the energy possessed by it is transferred to air and in overcoming friction at the point of suspension. Thus, the energy of the pendulum is dissipated in air. The law of conservation of energy is not violated because the energy merely changes its form and is not destroyed.