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Homework Help: Acceleration of a pulley. Torque, and moment of inertia.

  1. Dec 3, 2009 #1
    1. The problem statement, all variables and given/known data
    There are two blocks, one of mass m1 and the other of m2. They are connected by a rope that runs over a pully of radius R and interia I. m2 slides down a frictionless incline at an angle theta, and m1 hangs virtically while being pulled up by the tension in the rope. Find the expression for acceleration.

    2. Relevant equations

    3. The attempt at a solution

    With the previous two equations I came to this conclusion...


    The right answer has I/R^2 instead of just I.

    I figured it would be just I because the mass of an object is its inertia.

    Howwoud I derive the I/R^2?
  2. jcsd
  3. Dec 3, 2009 #2


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    Homework Helper

    How did you get an equation with an "I" when your initial equations:


    didn't have an "I" anywhere? Anyhow, for mechanics problems, you should always draw free-body diagrams for every object (including the pulley!), write down Newton's second law for each object, and then solve the resulting equations. That way, you'll see very clearly where the I/R^2 came from.
  4. Dec 3, 2009 #3
    I got the I from thinking that I of the pulley would be the same thing as mass for the blocks because isn't mass just the inertia of an object? and I put my net force over the total mass.

    I think I may have figured it out but I also may be making a mistake here...

    T1 and T2 are equal to the tensions on each side, and alpha=a/R





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