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Homework Help: Collision and Impulse

  1. Mar 29, 2009 #1
    1. The problem statement, all variables and given/known data
    Until his seventies, Henri LaMothe excited audiences by belly-flopping from a height of 12 m into 30 cm of water. Assuming that he stops just as he reaches the bottom of the water and estimating his mass, find the magnitude of the impulse on him from the water.

    2. Relevant equations
    v^2 = vo^2 +2ay
    j = pf - pi
    p = mv

    3. The attempt at a solution
    y = 12.3
    a = 9.8

    from equation 1, Vf = 15.53 m/s

    from equation 2 and 3 J = 15.53m -0m = 15.53m

    Help please.
  2. jcsd
  3. Mar 29, 2009 #2


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    What is his mass or what is it you estimate his mass to be?

    You've calculated the Δ of velocity, but j = Δp

    One small observation though. The Δp is taken as he contacts the water to at the bottom? That is where is p_final is taken. So wouldn't his initial v be based on a drop of 12m, not 12.3?
  4. Mar 29, 2009 #3
    I'm not sure what you mean by the second part of your comment. For Δp, v1 = 0, vf = like you said at the bottom. So the total height he travels is 12 + 0.3 from depth of water. I don't understand what you're trying to say.
  5. Mar 29, 2009 #4


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    That is the total distance of travel. But doesn't the question ask what is the impulse from the water. Wouldn't that be measured from the initial momentum at 12m? to the 0 velocity at the bottom?
  6. Mar 29, 2009 #5
    Okay but now I get Δp = 0 - 15.33(65) where 65 is estimated mass
    = -996.9 Ns

    The book shows the answer as 1.0 x 10^3 to 1.2 x 10^3... positive values. =S
  7. Mar 29, 2009 #6


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    What is positive y?

    It's not (0 - (-15.33)) ?
  8. Mar 29, 2009 #7
    ahh okay, i had the acceleration and y both as positive!

    thank you! =)
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