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Rotational Form of Newton's Second Law - Help

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data

    A turntable must spin at 33.3RPM (3.49 rev/s) to play an old fashioned vinyl record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 2 revolutions, starting from rest? The turntable is a uniform disk of diameter .305m and mass 0.22kg.

    2. Relevant equations

    I = 0.5MR²
    [itex]\tau[/itex] = [itex]\alpha[/itex]I
    [itex]\alpha[/itex] = (ωf-ωi)/t

    3. The attempt at a solution

    I=(0.5)(0.22kg)(.1525²)=2.56x10^-3 kgm²
  2. jcsd
  3. Nov 27, 2011 #2


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    There's a little problem with your conversion of units for ω. Instead of "rev/s", it should be 3.49 ___/s (?)
    Yup, that's I.

    Your book should have even more relevant equations for rotational motion. You want one that involves θ, so that you can use the information that it takes 2 revolutions to get the turntable up to speed. You can check in your textbook for the full list of equations.
  4. Nov 27, 2011 #3
    Starting from rest at a point O, let's call it, the motor supplies a torque so that by the second time we pass O, the angular speed is 3.49 rev/s. Based on this, you can use one of the kinematics equations (re-vamped into their respective rotational forms) and then incorporate the mass of the disc to find the torque.
  5. Nov 27, 2011 #4
    Yes, it is 3.49 rad/s, sorry :P

    Anyways, I found out the answer. My problem was that I didn't know theta was used as the 2 revolutions.
  6. Nov 27, 2011 #5


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    Okay, glad it worked out.
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