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Homework Help: Bungee jump- hard

  1. May 24, 2010 #1
    The girl is going on a bungee jump. She weighs 65 kg. The rope is 11m. In total, she fell 36 m. The first 11 m is free fall, and another 25m is simple harmonic motion.

    1) k of rope
    2)max spped
    3)what point when she has max speed
    4)amplitude of rope
    5)how long does it take

    2. Relevant equations
    vf^2= 2ax
    delta L= mg/k
    _/(k/m) A- square rt of k/m times A

    3. The attempt at a solution
    initial velocity = 0
    acceleration= 9.8m/s^2
    1) k of rope
    i got k for 25.48

    2)max spped
    max speed for free fall= 14.7m/s
    V_max for rope= 15.65m/s
    So do I add them up or just 15.65??!

    3)what point when she has max speed

    4)amplitude of rope

  2. jcsd
  3. May 24, 2010 #2
    why don't you show us how you solved each step, starting with the first one?
  4. May 25, 2010 #3
    1) i used deltaL= mg/k

    k= 25.48

    2) V_f^2=2(9.8)(11m)
    V_f=14.7m/s for free fall
    V_max= square rt of k/m *A
    so sqr (25.48/65) *25= 15.65 m/s

  5. May 25, 2010 #4
    ok, for part 1 you have it right, but try using a more accurate value for g = 9.81 m/s2. It will actually change your final answer.

    For part two, when do you expect the person to reach her final speed? And also, how did you derive "V_max= square rt of k/m *A"?
  6. May 25, 2010 #5
    V_max when the rope reach maxium stretch.
    I didnt derive it, it was a formula.
  7. May 25, 2010 #6
    oh i see what you meant now. but using kinetic and elastic energy relationships only holds when there is only an elastic force present for a given period of time. Is this the case?

    And plus, try to picture the scenario visually: once the elastic of the bungee jump is full stretched, what would you expect to happen? What would you guess her speed to be?
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