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Homework Help: Conservation of energy

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data
    A bungee jumper of mass 64.5 kg (including safety gear) is
    standing on a platform 48.0 m above a river. The length of
    the unstretched bungee cord is 10.1 m. The force constant
    of the cord is 65.5 N/m. The jumper falls from rest and just
    touches the water at a speed of zero. The cord acts like an
    ideal spring. Use conservation of energy to determine the
    jumper’s speed at a height of 12.5 m above the water on
    the first fall.

    2. Relevant equations

    3. The attempt at a solution
    so i did mgh = 1/2mv^2 + 1/2kx^2 and solve for v but somehow i am not getting the answer 6.3m/s
  2. jcsd
  3. Nov 14, 2011 #2


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    Homework Helper

    You may have substituted in some incorrect values, or you may have "mis-used" you calculator.

    Show us your substitution / simplifications so we can see what went wrong.
  4. Nov 14, 2011 #3
    this is what i did

    Attached Files:

  5. Nov 14, 2011 #4


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    Homework Helper

    Two problems:

    When 12.5 m above the water, the jumper has not descended 48m, so you h value is not appropriate, and the bungee has not been stretched 12.5 m, so your x value is not appropriate.
  6. Nov 14, 2011 #5
    12.5 above the water meaning 48 subtract 12.5 = 35.5m for the height?
    i dont know how much it stretched by am i suppose to calculate it?
  7. Nov 14, 2011 #6
    thx got the answer now
    Last edited: Nov 14, 2011
  8. Nov 14, 2011 #7


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    if it was 10.0 m long to start with it will have stretched 25.5m

    edie: the h value is 35.5, yes. best called the change in height.
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