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Acceleration in an Atwood's Machine

  1. Apr 14, 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

    I got the equation a=g*m1-m1/m1+m2 but it said this was incorrect because i need to function in M, the mass of the pulley. I am not sure how to incorporate that though.

    I also tried a=g*m1-m2/m1+m2+I/r^2
    it said this was incorrect also
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 14, 2012 #2

    jambaugh

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    Science Advisor
    Gold Member

    You need the moment of inertia of the pulley. For a disk it's 1/2M r^2 .
    Note that for a point mass rotating around an axis the MoI is mr^2
    A trick then is to treat the pulley as if it were a point mass with the same moment of inertia. You can then ignore the fact it rotates and treat it as an added mass.
     
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