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Homework Help: Two pendulums banging into each other problem.

  1. Aug 20, 2013 #1
    1. The problem statement, all variables and given/known data

    A system contains two metal balls hanging from strings of length !.0 m. Pendulum A has a mass of 2.0 kg and B has a period of 2.0s. They are released from the same height at the same time and collide right in the middle of their arcs. If pendulum A comes to a complete stop while B moves backwords with half of its original speed, what is the mass of pendulum B?

    2. Relevant equations

    Conservation of momentum.

    3. The attempt at a solution

    My first thought was the conservation of momentum. 2Va+MbVb=-(1/2)MbVb. Of course theres 3 variables so i couldnt solve it but then i thought that since the balls were colliding in the middle of their arcs that maybe they're velocities were the same. So Va=Vb. From that logic I get that the mass of B is -4/3 which obviously makes no sense..... Im not sure what im doing wrong. Im gratful for any help. Thanks in advance
  2. jcsd
  3. Aug 20, 2013 #2


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    The momentum is a vector. The balls move in opposite directions just before they collide. Va is not equal to Vb.

  4. Aug 21, 2013 #3
    Use the equation for the period of a pendulum to help you find Vb.
  5. Aug 21, 2013 #4


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    The time taken is irrelevant. You cannot deduce any actual velocity since you do not know the original height. ehild's hint is the way forward.
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