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Homework Help: Torque and Rotational Acceleration

  1. Aug 3, 2016 #1
    1. The problem statement, all variables and given/known data
    A 0.23-kg turntable of radius 0.31 m spins about a vertical axis through its center. A constant rotational acceleration causes the turntable to accelerate from 0 to 26 revolutions per second in8.0 s.

    Calculate the torque required to cause this acceleration.
    2. Relevant equations
    τ = α*m*r^2
    where τ is torque, α is rotational acceleration, m is mass, and r is the radius.

    3. The attempt at a solution

    I got the first portion of the question right which was to find α (20 s^-2), then I plugged the rest of the numbers into the equation and got τ = 20*0.23*0.31*0.31 = 0.44 but this was wrong. Where am I going wrong ? Thanks
  2. jcsd
  3. Aug 3, 2016 #2


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    The equation you are using is only applicable to a point mass ##m##, situated at distance ##r## from the axis of rotation. This problem is about a disc, not a point mass.
    You need to calculate the moment of inertia of the disc that is the turntable.
    If you don't have a formula, you could look up 'list of moments of inertia' on Wikipedia.
  4. Aug 3, 2016 #3


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    Andrew has identified the major error, but there is also a minor one. When using an intermediate numerical result as input to the next stage (the angular acceleration in this case) you should keep an extra significant digit to avoid accumulation of rounding errors.
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