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Homework Help: Not sure where to start

  1. Dec 14, 2007 #1
    [SOLVED] Not sure where to start....

    10. A playground merry-go-round is mounted on a frictionless axle, so it can only rotate
    in the horizontal plane. This object has a moment of inertia about the axle of I=50 kg m2,
    and it has a diameter of 2.2 meters. Initially it is turning at a constant rate so that it
    completes one revolution in 0.5 seconds. Standing next to it, you carefully place a block
    of mass m= 5.0 kg on the rotating table, so that it sits a distance r=1.0 m from the center.
    How long does it take to complete one rotation now?
    1. 0.5 s
    2. 0.55 s
    3. 0.59 s
    4. 0.63 s
    5. 0.67 s
    6. 0.71 s

    all i have is that i found wi = 12.57 rad/s...but i don't know what to do after that?
     
  2. jcsd
  3. Dec 14, 2007 #2

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    Can you think of any quantity which will remain conserved?
     
  4. Dec 14, 2007 #3
    The moment of inertia?
     
  5. Dec 14, 2007 #4

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    No, because the mass distribution has changed after you put the mass on it.

    If there's no external torque on a body, then the angular momentum remains the same. So, you should find the initial and final angular momenta.
     
  6. Dec 14, 2007 #5
    so initial and final angular momentum should equal since there is no external torque on the system right?
     
  7. Dec 14, 2007 #6

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    You said it.
     
  8. Dec 14, 2007 #7
  9. Dec 14, 2007 #8
    I do have one more question...how do u recalculate the new moment of inertia?
     
  10. Dec 14, 2007 #9

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    MI of the mass = mass*(dist from axis)^2
    Final MI = MI of disk + MI of the mass
     
  11. Dec 14, 2007 #10
    okay, thanks
     
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