NCERT Questions

Class 9
Chapter 11 Class 9 - 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?

A pendulum is a body which hangs from a fixed point and shows back and forth motion.

When bob is on right side, not released

When the bob of Pendulum is pulled to right side and not yet released

Since it has some height and it is not moving

It has Potential Energy but not Kinetic Energy

(Suppose Potential Energy = 10 J, Kinetic Energy = 0 J, Total Energy = 10 J)

When bob is released

When bob is released from hand, the bob loses potential energy and gains kinetic energy

(Suppose Potential Energy decreases to 8 joules, Kinetic Energy becomes 2 joules, Total = 10 Joules)

When it reaches center position

It has only kinetic energy but no potential energy

(Suppose Potential Energy = 0J, Kinetic Energy = 10J, Total Energy = 10J)

When bob goes left

This kinetic Energy makes the bob move towards left side

As it moves upwards towards other side, kinetic energy decreases and potential energy increases

(Suppose Potential Energy = 2, Kinetic Energy = 8J, Total Energy = 10J)

When bob reaches max left side

It soon reaches a point where potential energy becomes maximum and kinetic energy becomes 0

(Suppose Potential Energy = 10J, Kinetic Energy = 0 J, Total Energy = 10J)

Hence, Total Energy in all cases remain constant at 10 Joules

Hence, the law of conservation of energy is not violated in case of a simple pendulum since the energy is always conserved.

Eventually the bob comes to rest

During each oscillation, some energy gets transferred to the air due to overcome air friction.

So, Kinetic Energy of pendulum is lost due to air friction and the pendulum stops