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Homework Help: Physics - Moments of Inertia, Angular Momentums etc HELP!

  1. Apr 16, 2007 #1
    1. The problem statement, all variables and given/known data

    A large disk of mass M and radius R is spinning like a CD of merry-go-round - that is, about an axis perpendicular to the plane of the disk. It is rotating at an angular velocity (omega initial). At some instant, a sphere of mass M/4 , which is initially not rotating, is dropped onto the disk at a distance of 3R/ 4 from the center. The sphere sticks to the disk and begins rotating with it. Find the final velocity of the disk-sphere combination.

    2. Relevant equations

    solid cylinder = I = 1/2 mr squared

    solid sphere - I = 2/5 mr squared

    (not sure if these are the correct inertia formulas)

    3. The attempt at a solution

    i have no idea...i don't even know how to start the problem....should i find inertia of disk first?
  2. jcsd
  3. Apr 16, 2007 #2


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    Total angular momentum I*angular velocity is conserved. You should treat the sphere as a point mass. You aren't given its radius - so you can't truly treat it as a sphere.
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