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Homework Help: Angular momentum, transfer of mass between two drums.

  1. Jun 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A drum of mass MA and radius a rotates freely with initial angular velocity ωA(0). A second drum with mass MB and radius b > a is mounted on the same axis and is at rest, although it is free to rotate. A thin layer of sand with mass Ms is distributed on the inner surface of the smaller drum. At t=0, small perforations in the inner drum are opened. The sand starts to fly out at a constant rate λ and sticks to the outer drum. Find the subsequent angular velocities of the two drums ωA and ωB. Ignore the transit time of the sand.
    (picture available if needed)

    2. Relevant equations

    Angular momentum L=Iω
    I for these drums , I=R2

    3. The attempt at a solution
    At t=0, L(0)=a2(MA+MSA(0).

    and at L(t) = a2(MA+MStA+b2(MBtB

    conservation of angular momentum for the system, L(0)=L(t).
    and put λt=MB and b=2a (because of answer hint).

    I arrive at,

    and answer to this hint is ωBA(0)/8

    which I would get if ωA(0)-ωA = 0

    Any suggestions what I'm missing?
    Last edited: Jun 6, 2012
  2. jcsd
  3. Jun 6, 2012 #2


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    Oooh, I like this problem! :smile: It's very insightful. By the way, I think you're doing just fine so far. :approve:

    Regarding that last statement, ωA(0)-ωA = 0, if correct it means ωA = ωA(0). In other words, if the statement is correct, it's implying that drum A never changes its angular velocity, and rotates around at a constant velocity ωA(0) whether the sand is flying out of it or not.

    The next step is to figure out whether that's actually true or not, and if it is true, to prove it (at least prove it to yourself).

    You know that the angular momentum of the drum A plus sand combination changes over time, but that might simply be because the combination's mass, thus moment of inertia, is changing, perhaps. But let's step away from angular momentum for a moment and focus on kinematics and Newton's laws of motion, as applied to angular motion.

    Ask yourself the following:
    1. What is Newton's first law of motion? (Be sure to use the angular motion equivalent.)
    2. What, if any, torques are acting on drum A?
    Last edited: Jun 6, 2012
  4. Jun 6, 2012 #3
    Thanks and right, exactly. I got that far, and then...
    When sand leave drum A, it doesn't exert any torque on drum A and drum A doesn't exert and torque on the rest of the system. If no forces act on the drum it will keep on spinning about the same angular velocity. (Newton.1)
    Is this enough of an argument?

    What threw me off abit is when I thought about the formulas

    And since,
    [tex]\frac{dI}{dt}=\frac{dm}{dt}+\frac{dR^2}{dt}[/tex] is not zero there would be a torque.

    But this would be the sand system, which then exerts this torque on drum B. :confused:

    This problem sure has alot of physics in it. :smile:
  5. Jun 7, 2012 #4


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    I think you mean (using the chain rule),

    [tex]\tau=\frac{dL}{dt}= \frac{d(I\omega)}{dt}= \omega \frac{dI}{dt}+I\frac{d\omega}{dt}[/tex]


    [tex]\frac{dI}{dt}=R^2 \frac{dm}{dt}+ m\frac{dR^2}{dt}[/tex]

    But even with that correction, this has me scratching my head too on how to model the sand mathmatically.

    I think we can convince ourselves that the drum A rotates at a constant velocity in a different way.

    Consider an ice-skater holding on to two weights, one in each hand, as she goes into a spin. She can change here angular velocity by extending her arms in and out. But now suppose she's spinning at some constant arm length extension (so her angular velocity is constant) and instantaneously releases both weights.

    Let's take this thought experiment a step further. Suppose that instead of holding on to each weight with her hands, the weights are attached to a string. The skater and the attached weights are all spinning around together, and the skater is lightly holding onto the center of the string with her finger and thumb (ignore gravity if you wish, or assume that she's spinning around in outer-space or something). Now she carefully cuts the string between her finger and thumb with a pair of scissors and the weights go flying off. I think you can see that it's safe to say that cutting the string in this case is not going to create a torque on the skater, and she will continue spinning at the same constant angular velocity.

    The sand does in fact create a torque on drum B, because when the sand strikes the inner surface of drum B it has a tangential component to it (which is true as long as the radius of drum A is greater than 0).

    [Edit: that is there is a torque on drum B for a > 0, *and* up to some limit for ωB. Eventually after enough time has passed, ωB approaches some limit, the torque on drum B will approach zero, thus ωB will approach some constant.]

    But as long as you're satisfied that drum A continues at a constant angular velocity, you can use your previous work with conservation of momentum to determine the motion of drum B. Conservation of moment (as you've already done in your original post) makes this problem a lot easier.
    I should say so. :smile:
    Last edited: Jun 7, 2012
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