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Homework Help: Jumping off a building

  1. Jan 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A 120 kg man jumps off of a 5 story building (~15 meters). How much force is acted upon his body once he reaches the bottom?
    Will he break any bones?


    2. Relevant equations
    X = (1/2)at^2 (since initial displacement and initial velocity are zero)
    so therefore:
    a = 2x/t^2
    F= ma

    3. The attempt at a solution
    x = 15
    so: a = 20/t^2

    F= 120* ( 20/t^2)

    How do i solve for time?

    thanks
     
  2. jcsd
  3. Jan 2, 2010 #2

    PhanthomJay

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    This problem is missing information. His acceleration is g as he is falling, so the force acting on him as he falls is just his weight. The force acting on him as he hits the ground is quite different, and that depends upon what he falls onto (a safety net or concrete pavement?). Is there more information given??
     
  4. Jan 2, 2010 #3
    It's easier than that. This is a free fall motion so the acceleration is constant.(gravity)

    Use newtons second law with this knowledge. (F = ma)
     
  5. Jan 2, 2010 #4
    A concrete sidewalk.
     
  6. Jan 2, 2010 #5

    PhanthomJay

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    You cannot determine how much force acts on him as he hits the sidewalk, unless you know, for example, if he lands on his feet, how much he bends his knees as he hits. If he falls stiff legged, the force would be enormous, and he'd more than bust a few bones; if he bends his knees and rolls as he falls, you'd have to calculate that distance over which the impact force acts, then look up how much impact force the bone can take before breaking. I'd guess that although the force would be less than the force from a stiff legged landing, the fall could still be lethal. Don't try this at home!
     
  7. Jan 2, 2010 #6
    So let's make the assumption that he lands on his feet and the surface area of each foot is half a square foot.
    Now is it more possible?

    thanks
     
  8. Jan 2, 2010 #7

    PhanthomJay

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    Come on, with a 45 foot jump onto concrete, all bets are off....
     
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