Physics question about conservation of energy?

Question

A 70kg diver steps off a 10m tower and drops, from rest, straight down into the water. If he comes to rest 5.0m below the surface, determine the average resistance force exerted on him by the water.

I'm pretty much bamboozled. If you could include the steps to get the answer, that would be awesome!
Thanks!

Answer 1

First, calculate the diver's gravitational potential energy at the top of the cliff. That's GPE = mgh, where m is mass, g is the acceleration due to gravity, and h is height. You've been given m, and g is 9.8 m/s^2. Note that h should actually be the height of the diver above the final stopping point, so it will be 15 m. After the diver enters the water, the resistance force does work against the diver's motion. In order for the diver to come to a stop, the work must be equal to the diver's energy, which doesn't change prior to entering the water. Work is equal to W = Fd, where F is force and d is the distance over which the force acts. So if you set the initial GPE equal to Fd, F will be your only unknown, since you know d = 5.