Question

Two boys were playing with a ball on a level ground. One of the boys throws the ball on the third floor of the building which is at a height of 5 m and the other boy manages to throw on the fourth floor which is at a height of 7 m from the level ground. Find the ratio of the work done by both the boys?

• A. 1 : 1
• B. 2 : 3
• C. 5 : 7
• D. 3 : 2

Answer 1

The correct option is C

5 : 7

The ratio of work done by both the boys is the respective change in gravitational potential energies of the ball. Taking the ground level as the reference, the change in gravitational potential energy is $m\times g\times h$ where m- mass, g- acceleration due to gravity and h- height from the earth surface. The change for one ball would be $m\times g\times 5$ and for the other it would be $m\times g\times 7$ after starting from a zero gravitational potential energy from the ground.
$\therefore$The ratio of the work done would be equal to

$\frac{m\times g\times 5}{m\times g\times 7}=5:7$