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High School Physics

Kinetic Energy Numericals Class 9

Question 1) A car is moving with a uniform velocity of 15 ms-1. What is the kinetic energy of a boy of mass 40 kg sitting in the car?

Solution: Velocity of the boy = velocity of the car = v = 15 m/s
Here, the mass of the boy m = 40 kg.
Kinetic energy possessed by the boy = (1/2) mv2 = (1/2). 40. 152 = 4500 J

Question 2) If two bodies of equal masses move with uniform velocities of ‘v’ and ‘3v’, what will be the ratio of their kinetic energies?

Mass of first body = m
The velocity of the first body = v
Mass of second body = m
The velocity of the second body = 3v

The ratio of kinetic energies of the two bodies
= Kinetic energies of the first body / Kinetic energy of the second body
=[ (1/2) mv2 ]/[ (1/2) m (3v)2 ] = v2 / 9 v2 = 1/9

The ratio of the kinetic energies = 1:9


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