1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    Ek=1/2mv^2
    Ep=mgh
    Ee=1/2kx^2

    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

    PeterO

    User Avatar
    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

    PeterO

    User Avatar
    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

    PeterO

    User Avatar
    Homework Helper

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook