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Intext Exercise 11.3

What is the kinetic energy of an object?

Kinetic energy is the energy possessed by a body by the virtue of its motion. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.

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Write an expression for the kinetic energy of an object.

If a body of mass m is moving with a velocity v, then its kinetic energy is given by the expression,

Its SI unit is Joule (J).

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The kinetic energy of an object of mass, m moving with a velocity of 5 m s−1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Expression for kinetic energy is 

m = Mass of the object

v = Velocity of the object = 5 m s−1

Given that kinetic energy,  = 25 J

(i) If the velocity of an object is doubled, then v = 5 × 2 = 10 m s−1.

Therefore, its kinetic energy becomes 4 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 4 = 100 J.

(ii) If velocity is increased three times, then its kinetic energy becomes 9 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 9 = 225 J.

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