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Homework Help: Example Problem in the book, why is tension ignored?

  1. Dec 15, 2014 #1
    1. The problem statement, all variables and given/known data
    An Atwood machine consists of two masses, M and m, which are connected by an inelastic cord of negligible mass that passes over a pulley. If the pulley has radius R and moment of inertia I about its axle, determine the acceleration of the masses M and m.

    2. Relevant equations
    torque = dL/dt
    L(angular momentum) = R x v
    L = Iω

    3. The attempt at a solution

    This is the solution in the book. I have no idea why tension of the string is ignored.

    L = (m + M)vR + I(v/r) <- this part makes sense to me
    torque = mgR - MgR <- this part does not make sense to me

    Following the Atwood machine, shouldn't the forces for both m and M be something like
    ΣF = F(tension) - mg = ma (differing signs depending on which is going down, of course)

    Instead, the book has it as ΣF = mg

    So then since torque is equal to RF, they get that torque = mgR

    But my idea is that torque in this case is equal to RF(tension of m) - RF(tension of M)
    which would mean that
    torque = R(ma + mg) - R(mg - ma)

    After this, we just plug torque into the t = dL/dt equation, and with the factored out velocity that we get from the total angular momentum, get acceleration from dv/dt and simple algebra reveals that the answer is a = (m-M)g/(m+M)+I/R^2.

    Any help would be appreciated!
  2. jcsd
  3. Dec 15, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The book is treating the two masses and pulley as a single system. The tensions in the string would be internal to the system, so they can be ignored.

    You can certainly break up the problem and treat the two masses and pulley separately, getting three equations. Then you can solve for the acceleration that way.
  4. Dec 15, 2014 #3
    Hm, I just solved it using t = Ia with the 2 forces. So does does mean that dl/dt can only be used from an inertial frame of reference? Or do dl/dt = ia?
  5. Dec 15, 2014 #4

    Doc Al

    User Avatar

    Staff: Mentor

    If you mean the two tensions, then that's fine. ΣT = Iα

    If you let L = the angular momentum of the pulley only, then dL/dt will equal Iα.
  6. Dec 15, 2014 #5
    Ok, thanks!
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