Chapter - 4 Work and Energy

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Work and Energy


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?


According to the "law of conservation of energy": Energy can be neither created nor destroyed. It can only be converted from one form to another. Consider the case of an oscillating pendulum.

When a pendulum moves from its mean position O to either of its extreme positions A or B, it rises through a height h above the mean level O. At the extreme point, bob comes to rest and the kinetic energy of the bob is transformed into potential energy.

As it moves towards point O, its potential energy decreases progressively and simultaneously, the kinetic energy increases.

As the bob reaches point O, its potential energy is converted to kinetic energy. At this point bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates. The bob does not oscillate forever. It comes to rest due to some friction in the air.

The pendulum loses energy overcoming this friction. After all the energy is lost, it comes to rest. The law of conservation of energy holds here as the energy lost by the pendulum to overcome friction is gained by its surroundings. Hence, the total energy of the pendulum and the surrounding system is conserved.

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