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Homework Help: Trampolines = Fun

  1. Apr 21, 2005 #1
    A 55.3 kg person jumps from a window to a fire net 21.9 m below, which stretches the net 1.04 m. Assume that the net behaves like a simple spring, and calculate how much it would stretch if the same person were lying in it.

    Here's what I did:

    using conservation of energy to get the velocity of the person right before it hits the net. (mgh = 0.5mv^2) Once I got the velocity, I found the acceleration caused by the net (deceleration if you will) (v^2=v^2 + 2ay). Then using that acceleration, I found the force of the net on the person (F=ma) Once I found that force, I used it to find the spring constant(k) of the "spring". (F=ky). Finally, using that k, I found the displacement of the "spring" caused by the force of just the person's weight (F=kx where F=55.3kg(9.8)) and got an answer, which was wrong.

    Can anyone help me?
  2. jcsd
  3. Apr 21, 2005 #2
    You forgot to take into consideration the gravitational potential energy differance while the person was on the trampolin.


  4. Apr 21, 2005 #3
    hmm I don't quite understand the formula you came up with. I understand what you are saying, but perhaps you could elaborate a little more on how you came up with that formula. Thanks.
  5. Apr 21, 2005 #4
    oh by the way, thanks for the help.
  6. Apr 22, 2005 #5
    Law of conservation of energy. The gravitational potential energy is converted into the energy of the spring.


    The person doesn't only fall 21.9 m, he also falls for an additional 1.04 m while he's streching the trampoline.
    Last edited: Apr 22, 2005
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