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Homework Help: Unknown unstretched elastic cord length

  1. Dec 3, 2004 #1
    A person jumps from a balloon 65m above the ground. He will stop 10m above the ground. A 5m length of the same cord stretches 1.5m with his body weight. He will drop from rest at the point where the top end of a longer section of the cord is attached to the stationary balloon. What length of cord should the jumper use?

    Is the spring or elastic constant (mg/1.5)? If it is how do I set up the equation?

    I know that mgyi=65m, and mgyf=10m.
  2. jcsd
  3. Dec 3, 2004 #2

    Doc Al

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    Staff: Mentor

    That's the spring constant for a 5m length of cord; the spring constant is inversely proportional to the cord length.

    The key to this problem is energy conservation: The initial gravitational PE is transformed to spring potential energy. (Spring PE is given by [itex]1/2kx^2[/itex].) Realize that both the maximum spring extension (x) and the spring constant are functions of the cord length: Set up the equation and you can solve for that cord length.
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