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Homework Help: Elastic Collision of a ball and wire

  1. Mar 23, 2010 #1
    1. The problem statement, all variables and given/known data
    An 10.0 kg ball, hanging from the ceiling by a light wire 125 cm long, is struck in an elastic collision by a 1.20 kg ball moving horizontally at 7.50 m/s just before the collision. Find the tension in the wire just after the collision.

    2. Relevant equations
    M1V1o + M2V1o = M1V2f + M2V2f

    3. The attempt at a solution
    (1.2 kg)(7.5 m/s) = (10 kg)(Vf)

    Vf = 0.9 m/s

    Fc = mV^2/R = (10 kg)(0.9^2)/(1.25 m) = 6.48 N

    Fc = T - mg
    6.48 = T - (10 kg)(9.8)

    T = 104.48 N -- Book says it's incorrect, what am i doing wrong?
    Last edited: Mar 23, 2010
  2. jcsd
  3. Mar 23, 2010 #2


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    Hi Quincy! :smile:
    Because although it has a purely horizontal velocity, it has a vertical acceleration. :wink:

    Use Ftotal = ma. :smile:
  4. Mar 23, 2010 #3
    I realized that right after i posted the problem :P, i edited it now but the answer's still incorrect.
  5. Mar 23, 2010 #4


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    Nooo … you've ignored the momentum of the small ball after the collision. :redface:

    Try again, using conservation of both momentum and energy. :smile:
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