When the pendulum bob is pulled (say towards left), the energy supplied is stored in it is the form of potential energy 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 such that in mean position, it has maximum kinetic energy, and Zero potential energy. As the pendulum moves towards extreme right, its kinetic energy changes into potential energy such that at the extreme position, it has maximum potential energy and zero Kinetic energy. When it moves from this extreme position to mean position, its potential energy again changes to kinetic energy. 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 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.