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Homework Help: Unequal Accelerations in Double Atwood Machine

  1. Oct 11, 2011 #1
    I have a simple question about the Double Atwood machine.


    I am able to follow the solution but one thing makes no sense to me. The accelerations for block A and block B are:



    Why aren't these equal? If they aren't, doesn't that mean the rope stretches? Or is this just an effect of assuming a massless rope?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 11, 2011 #2
    In the few minutes since posting this, I have realized my mistake.

    a_b and a_a are not equal because that pulley is attached to a second pulley. When that pulley A and B are attached to moves up and down, it will add a term to both a_b and a_a, making them unequal.
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