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Homework Help: Angular Velocity Of A Sphere Rotating Under Gravity

  1. May 5, 2015 #1
    1. The problem statement, all variables and given/known data
    A solid sphere of mass m and radius a can rotate freely about a point A on its surface. The sphere is held initially at rest with the line OA through A and the centre of the sphere O horizontal and is released under gravity. Find the angular velocity of the system when OA first becomes vertical.
    (You may assume that the moment of inertia of the sphere about an axis through O is (2/5)ma^2.)

    2. Relevant equations
    energy change
    .5*Iz*w^2 = mgh

    where Iz is the moment of inertia about z
    w is angular velocity
    mgh potential energy

    3. The attempt at a solution
    parallel axis theorem
    Iz = (2/5)ma^2 + ma^2 = (7/5)ma^2

    energy change

    .5*Iz*w^2 = mgh (where h is the change in height of centre of mass)

    (7/10)*m*a^2*w^2 = mga


    w = (100/(7a))^.5

    I'm not sure how to find a or if I even have enough information to solve for a.

    Should I try angular momentum?
  2. jcsd
  3. May 5, 2015 #2


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    Homework Helper
    Gold Member
    2017 Award

    Your work looks good. Perhaps they just want the answer to be expressed in terms of the radius a. If so, it might be preferable to also leave g in symbolic form rather than substitute a value for g. That way, your answer will be correct for any system of units (as long as you use radians for angle).
  4. May 5, 2015 #3
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