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I'm missing something - help

  1. May 24, 2004 #1
    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. jcsd
  3. May 25, 2004 #2
    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
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