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Homework Help: Angular momentum; turntable problem

  1. Jan 15, 2013 #1
    1. The problem statement, all variables and given/known data
    A 1.7 kg , 20cm--diameter turntable rotates at 140rpm on frictionless bearings. Two 480g blocks fall from above, hit the turntable simultaneously at opposite ends of a diameter, and stick.

    What is the turntable's angular velocity, in rpm , just after this event?

    2. Relevant equations
    L = Iω
    Where L is angular momentum.
    L_before = L_after
    3. The attempt at a solution
    I'm not sure how to calculate the moment of inertia after the masses have landed on the turntable. for the turntable i used the equation I= .5MR^2 . Do I use the same equation for the two additional masses as well and add them to the moment of inertia of the turntable? or simply add the mass to the equation. (.5*(M_table + M_1 + M_2)*R^2)*ω or (I_table + I_m1 + I_m2)ω?
  2. jcsd
  3. Jan 15, 2013 #2


    User Avatar

    Staff: Mentor

    Treat them like point masses located at distance D/2 from the center of rotation.
  4. Jan 15, 2013 #3
    Thank you sir, that did it
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