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Homework Help: Newtonian - Force, Gravity, and Bungee Jumping

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data
    A person weighing 55kg jumps off of a bridge with a bungee cord attached. The problem seeks to find the bungee cords Force vector when the acceleration is 7.8m/s2 in the downward direction.

    m = 55kg
    a1 = g = 9.8m/s2

    To find:
    F1 when a2 = 7.8m/s2 downward.

    I've specified my grid as [tex]\uparrow[/tex]+ [tex]\rightarrow[/tex]+ so the accelerations are technically negative.
    2. Relevant equations
    [tex]\Sigma[/tex]F = ma

    3. The attempt at a solution
    [tex]\Sigma[/tex]F = ma
    [tex]\Sigma[/tex]F = -418N

    [tex]\Sigma[/tex]F = F1 - F2
    [tex]\Sigma[/tex]F = 539N - 418N
    Force of bungee cord acting = +121N
  2. jcsd
  3. Oct 7, 2009 #2
    Why did you subtract the forces?
  4. Oct 7, 2009 #3
    The 539N was the Force with F=ma and a = g. I think I forgot to write that.
    The -428N was the Force with F=ma and a = 7.8m/s2.

    My inclination was to subtract the forces to get the force required by the bungee cord to slow the decent to the 7.8m/s2
  5. Oct 7, 2009 #4
    Well since gravitational force is downwards and he is travelling downwards.......
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