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!