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Angular momentum question (from kleppner's book).

  1. Feb 10, 2007 #1
    question number 6.2 in page 279:
    A drum of mass M_A and radius a rotates freely with initial angular velocity w_A(0). A second drum with mass M_B 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 M_s is distributed on the inner surface of the amaller drum. At t=0, small perforations in the inner drum are opened. The sand starts to fly out at a constant rate [tex]\lambda[/tex] and sticks to the outer drum. find the subsequent angular velocities of the two drums w_A and w_B. ignore the transit time of the sand.

    ok obviously, we have conserved angular momentum and conserved linear momentum (no external force).
    i.e I_Aw_A+I_Bw_B=I_Aw_A(0)
    and (M_A+M_s)v0=(M_A-[tex]\lambda[/tex]t)(w_A)a+(M_B+[tex]\lambda[/tex]t)(w_B)b
    where v0=w_A(0)*a
    but i dont get it from the final answer which is the answer clue:
    if lambda*t=M_b and b=2a then w_B=w_A(0)/8.

    am i missing something here?
    btw, the moments of inertia here are of a uniform think hoop, right?
    which MR^2.
  2. jcsd
  3. Feb 12, 2007 #2
    no one can help me on this?
  4. Feb 12, 2007 #3
    I get that answer only if M_A=0. I must be missing something, or I would have at least tried.
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