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Dreaded Atwood Problem

  1. Apr 19, 2005 #1
    I have been working on this problem off-and-on for a couple of days now and cannot seem to get the correct answer. The problem is an Atwood machine problem. The following are the variables:

    Mass of pulley = .20 kg
    Radius of pulley = .15 m
    Clockwise frictional torque = .35 m*N
    Mass 1 on right side = .40 kg
    Mass 2 on left side = .80 kg

    The problem wants to know the acceleration of the masses. The correct answer is 1.2 m/s^2.

    My thought was to set the net torque of the system equal to moment of inertia times angular acceleration. Take that number and multiply by the radius to get the tangential acceleration. I have tried the following to get the angular acceleration:

    T2 - T1 - Tf = Iα

    Which equates to:
    m2gR - m1gR - Tf = Iα (solve for α)

    With the above formula, which must obviously be the wrong approach, I cannot seem to match the answer in the back of the book. What am I missing here?
  2. jcsd
  3. Apr 19, 2005 #2


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    Staff: Mentor

    Ah, the dreaded Atwood machine! In our General Physics lab, we call it the "toe-cruncher." :eek:

    [itex]T_1 = m_1 g[/itex] and [itex]T_2 = m_2 g[/itex] only if the acceleration is zero. Do a free-body diagram for each mass to figure out the tension in each string in terms of the acceleration.
    Last edited: Apr 19, 2005
  4. Apr 19, 2005 #3
    If you search the threads, I'm very sure you will find that someone else has already dealt with this kind of problem (I know because I've posted such problems before).
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