Chapter 10

NCERT
Class 9
Science
Solutions
15. 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?

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?

Answer:

When the bob of a simple pendulum is draw to one side and released, it starts oscillating. Let the bob be released from point B, then the mean passes through position. A to reach point C. Again reaches back to B from C. Thus, point B has maximum potential energy. There is no kinetic energy here. Again when the bob reaches the mean position A, it has maximum kinetic energy. There is no potential energy.

In the same way, when the bob reaches again from point A to point C, then this point has maximum potential energy. There is no kinetic energy. Hence, maximum potential energy is converted into maximum kinetic energy and maximum kinetic energy into maximum potential energy which explains the law of conservation of energy. Due to the friction of air, the bob gradually comes to rest and all the kinetic energy is converted into heat energy. That is, energy is transformed from one form to another.

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