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Finding velocity before impact

  1. Sep 15, 2009 #1
    1. The problem statement, all variables and given/known data

    A person jumps from the roof of a house 3.4 meters high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of 0.70 meters. If the mass of his torso (excluding legs) is 41 kg.

    A. Find his velocity just before his feet strike the ground.

    B. Find the average force exerted on his torso by his legs during deceleration.

    2. Relevant equations
    I can't even seem to figure that part out. Help please?

    3. The attempt at a solution
    I don't know how to start this at all.
  2. jcsd
  3. Sep 15, 2009 #2


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    Start with step 1. What his velocity when he reaches the ground?
  4. Sep 15, 2009 #3


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    hint: U=mgh=.... [use the law of conservation of energy to fill the gap]
  5. Sep 16, 2009 #4
    his velocity would be 8.14 m/s. i think
  6. Sep 16, 2009 #5
    Remember mv^2 * 0,5 = E, that should help you solve the first. For the other remember that Energy is force times distance.
  7. Sep 16, 2009 #6
    the force would be gravity in this case right?
  8. Sep 16, 2009 #7


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    Right. Your answer in #4 is correct. (I get 8.16 m/s, using g=9.8 ms-2)

    Now... how do you plan to approach part B?
    Last edited: Sep 16, 2009
  9. Sep 16, 2009 #8
    Not exactly, the force comes from the legs, the muscles have to work. (I'm a little unsure though.) In the first question (A) potential energy is transformed to kinetic energy, in the second the same energy is "soaked up" by the legs.
  10. Sep 16, 2009 #9


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    Yes, the force for part A is gravity. The force for part B is applied to the torso through the legs to decelerate.
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