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Homework Help: Rotational Problem

  1. Jun 16, 2011 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 = 2.00 kg and a block of mass m 2 =
    6.00 kg are connected by a massless string over a pulley
    in the shape of a disk having radius R = 0.250 m and
    mass M = 10.0 kg. These blocks are allowed to move on
    a fixed block – wedge of angle 30.0°, as shown in
    Figure P10.39. The coefficient of kinetic friction for
    both blocks is 0.360. Draw free-body diagrams of both
    blocks and of the pulley. Determine (a) the acceleration
    of the two blocks and (b) the tensions in the string on
    both sides of the pulley.

    Diagram: http://www.webassign.net/pse/pse6_p10-37.gif

    2. Relevant equations

    3. The attempt at a solution
    Now I know how to solve this problem, the only thing I'm stuck on, is the pulley. I know that you have to make a substitution for the acceleration using:
    Torque = (T1 + T2)R = I(alpha)
    T1 + T2 = I(a/R^2).
    The only thing I don't understand is why T1 is negative and T2 is positive (T1 being the tension affecting m1, and T2 affecting T2). By R.H.R, torque1 = T1xR = out of page, which I just assumed should be positive, and thus T2 should be negative, but I only get the right answer if I have T2 - T1 = I(a/R^2), and not T1 - T2 = I(a/R^2). I don't think I'm missing a sign anywhere else, this is kind of a popular problem, and have compared my solutions to others.

    Any help would be appreciated.
  2. jcsd
  3. Jun 16, 2011 #2
    nvm. Clockwise = negative
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