Concepts

Class 9
Chapter 11 Class 9 - Work and Energy

What is Pendulum?

It is a body which hangs from fixed point

It swings back and forth (move form left to right and right to left)

It contains a heavy metal ball which is tied to a string

This string is attached to a support such that metal ball hangs down

What is Conservation of Energy

When energy changes form from one form to other

Total Energy remains constant

There is no loss or gain of energy

How does Pendulum work on Principal of Law of Conservation of Energy

When the ball of Pendulum is pulled to one side and not yet released

It has Potential Energy but not Kinetic Energy

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

When ball is released from hand,the ball 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)

This kinetic Energy makes the metal ball move towards other 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)

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

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

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

Questions

NCERT Question 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? Is it a violation of the law of conservation of energy?

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