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?

Find the height it would have reached without air resistance.

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Use the motion formula :v^2 = u^2 + 2as for that,

v - final velocity (zero)
u - initial velocity
a - acceleration due to gravity (-9.81 because it is against the initial velocity)
s - the estimated height reached.
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You can find that from the energy formula too; E_k = \frac12 mv^2. With this, find the total energy of the ball, and equate that energy with that of potential energy,E_p = mgh to find h directly.
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Then, find the actual height the ball reached using the percentage given.

Use the potential energy formula, E_p = mgh to find the energy of the ball at both heights. The difference is the work done by air resistance.

Use again the law of conservation of energy. The work done by air resistance will be the same. This time, use the formula E_k = \frac12 mv^2 for kinetic energy to find the speed v of the ball on hitting the ground, that is when all the remaining potential energy has been converted to kinetic energy.