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Homework Help: Conservation of energy problem

  1. Apr 20, 2008 #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.



    3. The attempt at a solution

    so I decided that Eg(at the top of the platform)=Ek+Ee+Eg'(12.5 above water floor)

    so for h initial i used 48m...h'= 12.5m and x(stretch of the cord)= 25.5m
    therefore
    mg(48)=.5k(25.5^2)+.5mv^2+mg(12.5)

    i solved for V and got 5.96m/s, the answer is 6.37m/s...please help!!
     
  2. jcsd
  3. Apr 20, 2008 #2

    Nabeshin

    User Avatar
    Science Advisor

    I think your error is merely computational. I get the stretch of the cord to be 25.4m, not 25.5, which apparently has a large difference because the value is squared.
     
  4. Apr 20, 2008 #3
    bingo!..thanks a lot!
     
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