# Homework Help: I'm missing something - help

1. May 24, 2004

### speg

I'm missing something - fixed :P

Ok, doing practice stuff over the summer ( expect more of these :P ) and anyway this should be easy but I can't get it to work.

A diver ( mass = 70 kg ) jumps off a board 10m above the water. If his downward motion is stopped 2.0s after he enters the water, what average upward force did the water exert on him?

Ok, I get his speed when he hits the water to be:
(2*9.8*10)^(1/2) = 14 m/s

Then it takes him 2s to stop... so we can find that acceleration:
a = 14/2 = 7

F=ma = 70*7 = 490 N

But the back of the book tells me it should be 1180 N.

Sorry about the presentation, I'll have to learn how to enter this stuff soon. :)

Haha, silly me forgot about gravity :O How do I keep all these things straight - there's so much stuff to worry about!

Last edited: May 25, 2004
2. May 25, 2004

### Cummings

yes, the force required is the force to act against gravity (70*9.8) plud the force required to act against the 14m/s motion (70*7)

so (70*9.8)+(70*7) = 1176 N