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Homework Help: Finding mass in a confusing problem

  1. Oct 14, 2005 #1
    I have tried many times to figure out the formula to solve this problem. I have drawn diagrams & searched through my book numerous times. If I just had the formula to solve it, I would be fine. I dont have a scanner so I cant show you all the diagrams & whatnot but here is the problem.

    " high jumper falling a 3.9 m/s, lands on a foam pit and comes to rest, compressing the pit a distance of 0.43 m. If the pit is able to exert an average fource of -1100 N on the high jumper in breaking the fall, what is the jumper's mass? "

    Please help...
  2. jcsd
  3. Oct 14, 2005 #2


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    The jumper has a velocity of 3.9 m/s just as they hit the foam, and comes to rest after travelling 0.43 m. So you need to work out the deceleration, or negative acceleration.

    You have,


    What eqn of motion connects all three ?
  4. Oct 14, 2005 #3
    Another way to look at it is, need m, have disp, veloc, and the force bringing the jumper to a halt. A quick substitution into an equation gives you what you need , but its essentially the same thing, just a different approach.
  5. Oct 14, 2005 #4
    Thanks, Ill try working with that. If I have any more questions, and Im sure I will, Ill be back.
  6. Oct 14, 2005 #5


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    If you come back: note that you have a force and a distance. What does that suggest? Once you have that, you have the speed with which the jumper strikes the pit - from that, you should be able to get the mass. Think energy.

    Note: Depending on how tricky you want to get, you might also take into account the change in gravitational potential energy of the jumper as she comes to rest. It shouldn't complicate the algebra by much.
  7. Oct 17, 2005 #6
    Ok Im back.
    Now Ive got it down to two answers but Im not sure if Im supposed to multiply or divide at a certain point.
    Is it - 0.43 * 3.9 = 1.677 * 1100 = 1845
    or is it - 0.43/3.9m/s = 0.1102 * 1100 = 121

    Or am I completely off?
    Please help..
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