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Homework Help: Bungee Jumper Physics Lab Question

  1. May 28, 2009 #1
    1. The problem statement, all variables and given/known data
    So I have a final lab in my college physics class. And the lab procedure is as follows:
    Note: The measured mass of the Bungee jumper is 0.25837 kilograms (258.37 grams).
    I have attached my data as an Excel file:
    View attachment AP Physics Post Lab.xls .
    My question is, what formula does k follow (it is most definitely not linear)?

    2. Relevant equations
    Thus, once I find k, it should hopefully be easy to compute this minimum height using energy considerations (please let me know if I am somehow wrong):
    [tex]U_i + K_i = U_f + K_f[/tex],
    Since the object is released from rest,
    [tex]mg(h_{min}-{L_0}) = mg(0.05)+0.5k(h_{min}-0.05-L_0)^2[/tex],
    where [tex]h_{min}[/tex] is the minimum height (the thing I need to calculate), m is the mass of the jumper = 0.25837 kg. [tex]L_0[/tex] is the initial length of the bungee which I measured to be 0.395 meters.

    3. The attempt at a solution
    In the attached Excel file I included a graph of the empirical computation of k (y-axis) versus the attached mass in kilograms (x-axis). As you can see, k asymptotically approaches 4 N/m, but is non-linear. I estimate k to be around 7 N/m if the bungee jumper is attached.
    Last edited: May 28, 2009
  2. jcsd
  3. Jun 8, 2009 #2


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    Science Advisor
    Homework Helper

    flouran: I don't think I would bother with computing k, for the main calculations, although k is informative (for information only).

    Your empirical data, although extremely coarse, suggests that a better regression would be P(x) = 17.24*x/(1 + 4.786*x - 1.886*x^2), where P(x) = applied force (N) as a function of x, and x = deflection (m). Thus, U11 + K1 = U12 + K2 + U22, where U1 = potential energy, and U2 = strain energy. Therefore,

    m*g*h1 + 0 = m*g*h3 + 0 + integral[P(x)*dx],

    where h1 = jumper initial height above ground, h3 = jumper final height above ground, and the integral is integrated from 0 to h2 - h3, where h2 = height of unstretched bungee cord lower end above ground (the unknown). Unless you can find an analytic solution to the above integral, which I did not attempt, you could solve the problem numerically, by trial and error, if you wish.
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