Physics help please?

Question

A 78.0 kg diver steps off a diving board and drops straight down into the water. The net force when in the water is an upward 1020 N. If the diver comes to rest 4.2 m below the water's surface, what is the total distance between the diving board and the diver's stopping point underwater?

Answer 1

When she steps off the diving board, the diver accelerates from zero up to a speed "v", and then hits the water.

At that point, she decelerates from "v" back down to zero again, in the space of 4.2 meters.

We know that her acceleration (deceleration) in the water is:
a = F/m = 1020N / 78.0 kg = 13.07 m/s²

From that (and knowing that the distance "d" is 4.2 m) you can figure out her speed at the water's surface, using this formula:

v = sqrt(a·d)
= sqrt(13.07 m/s² · 4.2m)
= 7.4 m/s

Next, now that you know her speed when hitting the water, you can figure out how high the diving board is, using this formula (it's basically the same as the other formula, but uses "g" instead of "a"; and "h" instead of "d"):

h = v²/(2g)

Finally, the answer is h+d.