When a pendulum bob is drawn to one side and allowed to oscillate, it follows the law of conservation of energy. It comes to rest because of air ____. The law is not violated when the pendulum comes to rest.

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When a pendulum bob is drawn to one side and allowed to oscillate, it follows the law of conservation of energy. It comes to rest because of air friction. The law is not violated when the pendulum comes to rest.
Air friction:
Air resistance is a kind of friction (a force that opposes the motion) which acts between the air and another object. This force is experienced by an object as it passes through the air. When a pendulum bob is raised to the right extreme, the kinetic energy of the bob is

zero

and the potential energy of the bob is

mgh

where m is the mass of the bob, g is acceleration due to gravity; h is the height to which the bob is raised. When the bob is released the speed of the pendulum increases as its height decreases hence the potential energy stored in the bob at the left extreme is converted into kinetic energy. At the mean position all of the potential energy is converted into (maximum) kinetic energy. Again at the left extreme all the kinetic energy is converted into the potential energy. The total energy remains constant if air friction is negligible and hence it follows the law of conservation of energy. Some part of kinetic energy is lost in overcoming the air friction and hence the pendulum comes to rest after a period of time. Since the energy is transferred and is converted to another form while overcoming the friction hence,
law of conservation of energy is not violated.
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