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Homework Help: Acceleration in a Weight Machine

  1. Apr 13, 2012 #1
    1. The problem statement, all variables and given/known data

    An Atwood's machine consists of two masses, m1 and m2, connected by a string that passes over a pulley.
    If the pulley is a disk of radius R and mass M, find the acceleration of the masses.
    Express your answers in terms of variables m1,m2, M, R, and appropriate constants

    2. Relevant equations


    3. The attempt at a solution

    this is an equation i found but i am not sure what to do with I
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 13, 2012 #2


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    hi eagles12! :smile:

    (try using the X2 button just above the Reply box :wink:)
    you mean you found it on a website? :rolleyes:

    (it doesn't look right, but anyway …)

    you need to call the tension T, and do F = ma twice (once for each mass), and τ = Iα for the pulley, and then eliminate T …

    what do you get? :smile:
  4. Apr 13, 2012 #3


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    It is not a good idea to find an equation without doing some work to arrive at the solution. Please make an attempt. The I for the pulley is the least of your problems.:frown:
  5. Apr 13, 2012 #4
    The correct answer can be found here:

    but I agree with the others you need to find a way to it, that's the point. My guess is that looking at the energy balance is a shorter way in this problem.
  6. Apr 13, 2012 #5
    Well i found that equation from a similar problem in my textbook and then altered it for my problem
  7. Apr 14, 2012 #6
    I am still having trouble figuring out how to use the M (mass of the pulley) because i need it in my equation.
    i tried a=g m1-m2/m1+m2
  8. Apr 15, 2012 #7
    I guess you are missing the pulley rotation equation: I [itex]\dot{\omega}[/itex] =(T2-T1)R
    where I is the pulley torque and equals MR^2, (omega)'=a/R. T1 and T2 are the tension forces on both sides. acceleration of m1 and m2 is a.
    Now you should have three equations: the second Law for m1, the second law for m2 and the rotation equation for the pulley.
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