Question

A person on a roof throws one ball downward and an identical ball upward at the same speed. The ball thrown downward hits the ground with 100 J of kinetic energy.
Ignoring air friction, with how much kinetic energy does the second ball hit the ground?

A) 200 J

B) 100 J

C) Less than 100 J

D) More than 200 J

E) none of the above

Answer 1

Answer:C

Explanation:

Given

Both ball are thrown simultaneously with same velocity

Suppose h be the height of roof and u be the initial velocity of balls

For ball thrown from room initial total energy is

$E_1=Potential\ Energy\ +\ kinetic\ energy$

$E_1=mgh+\frac{1}{2}mu^2$

Now this ball potential energy convert to kinetic energy and add up in available kinetic energy to give 100 J at bottom

For second ball, energy at bottom is equal to kinetic Energy of ball

Therefore this energy remains constant and only converts to Potential Energy as ball moves upward

but as soon as ball hit the ground energy will be less than 100 J because first ball has sum of Potential and Kinetic energy .