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Homework Help: Turntable Rotating on Shaft Problem

  1. Nov 13, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-11-13_19-23-39.png

    2. Relevant equations
    ωf = ωo = αt

    ∑τ = Iα

    3. The attempt at a solution

    1) Find the acceleration

    ωf = ωo + αt

    fo) / t = α = -0.231 rad/sec

    2) Find ∑τ

    ∑τ =Iα

    I assumed that the only torque acting on the turntable was the torque due to friction

    τfriction = (1/2)MR2(-0.231) = -.00312 N*m
     
  2. jcsd
  3. Nov 13, 2016 #2

    TSny

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    Looks OK so far. How do you get the friction force from the torque?
     
  4. Nov 13, 2016 #3

    gneill

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    I didn't see a justification for ignoring the moment of inertia of the shaft (I'm assuming that the M and R above are for the disk).
     
  5. Nov 14, 2016 #4
    So if I factor in the moment of inertia of the shaft I get the ∑τ = Itotalα

    => [(1/2)MR2 + (1/2)mr2 ](-0.231) = -0.00312 N*m

    => Ffriction = ∑τ / (R+r) = -0.020 N

    The correct answer should be 0.52 N so I'm not sure how I'm so far off.
     
  6. Nov 14, 2016 #5

    TSny

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    Why use R + r ? Note that the brake pads are applied to the shaft.
     
  7. Nov 15, 2016 #6
    Okay if I use r instead of R + r I get the right answer. Thanks!
     
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