Question

A ball is dropped from a height of $100m$. At the surface of the earth $20\%$ its energy is lost. To what height will the ball rise?

• A. $80$m
• B. $20$m
• C. $40$m
• D. $60$m

Answer 1

The correct option is A $80$m

Given,

$h=100m$

let $m=$ mass of ball.

The potential energy at height $h=100m$ is

$PE=mgh=100mg$. . . . . . .(1)

At the surface of the earth, 20% of the initial energy is lost. So now the potential energy will be

$100mg-20mg=80mg$

Now $h^{\prime }$ be the maximum height reached by the ball after striking the surface of the earth

The potential energy at the height $h^{\prime }$ will be

$80mg=mg{h}^{\prime }$

$h^{\prime }=80m$

The correct option is A.