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Homework Help: Angular Velocity of a turntable-man system

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data
    A turntable rotates about a fixed vertical axis, making one revolution in 10s. The moment of inertia of the turntable about the axis is 1600kgm^2. A man, James Bond, of mass 120kg, is at the center of the turntable and begins to run out along a radius of the device. Treat the man as a point object.


    2. Relevant equations
    L = Iw


    3. The attempt at a solution
    There are three parts. The first part asks for the initial angular velocity, which is simply converting 1rev/10s into rad/s. The second part asks for the initial angular momentum, which would be L = Iw?

    The third part is asking for the angular velocity of the turntable-man system when the man is 2m from the center of the turntable. It also says at this point the man is at rest with respect to the turntable.

    Now I run into my problems. I'm not too sure about how to calculate this, but one method I'm thinking of is recalculating the inertia to be 1600kgm^2 + mr^2 where m and r are referring to the man (so 120kg and 2m respectively). This would give an inertia of 1840kgm^2.

    I thought about using L = r (cross) p, but solving for the linear momentum would mean finding Vtan = wr? How would I find w in this case?
     
  2. jcsd
  3. Dec 16, 2008 #2

    rl.bhat

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    Since there is no external torque acting on the tern table, angular momentum remains constant.
     
  4. Dec 16, 2008 #3
    I was considering that approach but wasn't sure. Thanks. However, my approach to the new inertia would be correct, I assume?
     
  5. Dec 16, 2008 #4

    rl.bhat

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