A 0.45-kg ball was kicked straight up with an initial speed of 18 m/s. The ball
Reached a height that is 80% of the height that would have been reached had
There been no air resistance.
(a) Use the law of conservation of energy to calculate the work done on the ball, on its way up, by the force of air resistance.
(b) With what speed did the ball hit the ground?

(a).
Calculate the height the ball would reach if there were not air resistance. (use
Find the 80% of this height.

Find the energy acquired by the ball going up without air resistance.
Find the energy acquired by the ball going up with air resistance.
Find the energy difference between those two.

You can check your answer with the motion formulas:
Work done is Force times displacement. You have the displacement (displacement is that which the wind caused, that is displacement without air resistance minus displacement with air resistance). Force is from F=ma.

(b) for this one, I'm not sure of how to do it. I better say nothing just in case I make a mistake.