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AP Physics Angular Motion

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  1. Jan 22, 2013 #1
    1. The problem statement, all variables and given/known data
    Okay so this is a multi part question. I think I am on the right track, but I am most likely wrong. Any help is good. If I know what to use for the equations I can usually solve it on my own.
    The question is:
    A child of mass 40kg is on a merry go round. The ride is a uniform disk of mass 150kg and radius 2m. The child is hanging onto a pole at a distance of 1m. Her father, mass 80kg pushes on the ride with a constant tangential force of 100N.
    A) Determind the rotational inertia of the system (child and ride)
    B) Determind the angular velocity in rpm of the child after 5s
    C) The man releases the ride and it continues at a constant angular velocity. If the child moves to the ouside of the ride, 2m away from the center, what will be the new angular velocity?


    2. Relevant equations
    A) This was the easy one. I=1/2mr^2
    B) I think this one is θ=(ω/2)t (its really ωfinal plus ωinitial but the initial ω would be zero, so I canceled it out.
    I doesn't specify the revolutions so I assumed that it was just one. (I hope that is right) But I think that would make θ=6.283rad
    C) I have no clue. I feel like it would be the same as B but distance, the only thing that changes, isn't a factor. Also it doesn't give a new time or anything.

    3. The attempt at a solution
    A) 360kgm^2
    B) 150rad/s if I did it right.
    C) No clue
     
  2. jcsd
  3. Jan 22, 2013 #2

    tms

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    What about the child?
    The question asks for ω, not θ.
    Something has definitely changed; see part A.
     
  4. Jan 22, 2013 #3
    Thanks for replying.
    For A, I used the mass of the child plus the mass of the ride for m. Is that right?
    For B, I just solved for ω, so it was really ω=2θ/t, so it was ω=2(6.283rad)/5s Is that right?
    For C, I'm still not getting it..
     
  5. Jan 22, 2013 #4

    tms

    User Avatar

    No. Disks and point masses (assuming the child as one) do not have the same moment of inertia.
     
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