A high diver of mass 65.0 kg steps off a board 10.0 m above the water and falls vertical to the water, starting from rest. If her downward motion is stopped 1.70 s after her feet first touch the water, what average upward force did the water exert on her?

Question

matt101 · Answer

Let's work backwards, because this question will take a few steps. We want to find a net force, and to do that we need to know the mass and the acceleration (remember, F = ma). The mass is that of the diver, which we're given in the question. Somehow we need to find the acceleration.

Acceleration is the change in speed over time. We know the time it takes for the diver to stop underwater (1.70 s). We also know the final speed underwater - it's 0 m/s because the diver is stopped. We need to find the initial speed underwater. However, keep in mind that the initial speed underwater is the same as the final speed the diver hits the water at! We can find this speed using this equation:

$$v_{f}^2=v_{i}^2+2ad$$
where v(f) is the final velocity when the diver hits the water, v(i) is the diver's initial speed outside of the water (0 m/s, since the diver isn't moving at the beginning of this situation), a is the acceleration outside of the water (9.8 m/s^2, the acceleration due to gravity since the diver is falling), and d is the distance covered (10 m).

Whew! Hopefully that made a bit of sense. If you need me to clarify something please ask!

matt101 · Answer

No problem! Just read my answer step by step and let me know which part don't you understand!

matt101 · Answer

Notice that the velocity is squared - so you need to square root your answer to get 14 m/s. That's the velocity at which the diver hits the water. Now looking at the underwater situation, you have the change in velocity (v(f)-v(i), or 0 m/s - 14 m/s) and the time it took for that change in velocity to occur (1.70 s). Now you can find acceleration! Then just use that to find the force!

anonymous · Answer

still doesnt work