Physics 20: Newtons 1st and 2nd Laws + Weight-mass

Question

A 70k swimmer falls off a 10m high diving tower. Determine the net force exerted by the water, if the water bringes the swimmer to a stop at 2.0s.

Answer 1

The potential energy that the swimmer has prior to falling is calculated as:

E_P = mgh = (70)(9.8)(10) = 6860 joules

Just at the point the swimmer is the enter the water, this potential energy has all been converted into kinetic energy and is calculated as:

E_K = (1/2)mv² =

We can calculate the velocity by plugging in 6860 joules as the E_K with m = 70 kg:

6860 = (1/2)(70)v²

v² = 196

v = 14 m/s

Using the Impulse-Momentum Theorem, the impulse equals the change in momentum.

Impulse equals force multiplied by time = F(2 seconds) = 2F

The change in momentum equals m(v_f - v_i) = 70(0 - 14)

So 2F = 70(0 - 14)

F = -490 N or 490 N in the direction opposite the entry into the water