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Homework Help: Impulse and Momentum of a belly flop

  1. Oct 29, 2008 #1
    1. The problem statement, all variables and given/known data

    A relative of yours belly flops from a height of 2.50 m (ouch!) and stops moving after descending 0.500 m underwater. Her mass is 62.5 kg. (a) What is her speed when she strikes the water? Ignore air resistance. (b) What is the magnitude of her impulse between when she hit the water, and when she stopped? (c) What was the magnitude of her acceleration in the pool? Assume that it is constant. (d) How long was she in the water before she stopped moving? (e) What was the magnitude of the average net force exerted on her after she hit the water until she stopped? (f) Do you think this hurt?

    So I'm stuck on C here, and I have a feeling that the solution is quite simple as well.

    2. Relevant equations
    J = F*t = Pf - pi
    Conservation of momentum...

    3. The attempt at a solution

    I already solved for the speed and the impulse.

    I found speed to be 7m/s and impulse to be -437.5 Kg m/s when she strikes the water. I surmised that when she stops her impulse must be 0... what do they mean by between when she hit the water and when she stopped? I'm guessing this is a change in impulse, which I suppose is exactly what I answered.

    Anyhow, I've been manipulating various equations trying to find acceleration in the pool. I know that time is not necessary to solve for acceleration, because the next question asks for the time.

    I tried using conservation of energy, conservation of momentum and impulse equations. I couldn't seem to isolate force without time.

    Any hints? Thank you in advance
  2. jcsd
  3. Oct 30, 2008 #2
    You mean when she stops her velocity is 0? The impulse is the change in momentum (i.e. between when she hit the water and when she stopped) so your reasoning here is right.

    Have you tried with the equations for one-dimensional motion?
  4. Oct 30, 2008 #3
    Funny you should suggest that. I actually right after posting. for some reason my units kept coming out in m^3/s^2... no idea why. I checked over it numerous times but found nothing
  5. Oct 30, 2008 #4
    Could you show us what you did? (step by step)
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