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Rotational Motion of a spun disk

  1. Nov 30, 2007 #1
    A 200 g, 20-cm-diameter plastic disk is spun on an axle through its
    center by an electric motor. What torque must the motor supply to take
    the disk from 0 to 1800 rpm in 4.0s? (Given: I = ½ MR2)

    Relevant equations
    I = 1/2 MR^2
    T = Ia

    The attempt at a solution
    v=0.02m * 1800 rev/min / 4 sec * 60 sec / 1 min * 2 Pi / 1 Rev
    v= 10 800 m/s

    a = how do i get this?

    I = 1/2 (0.2kg)(0.1)^2
    I = 0.001

    T = Ia
    T = (0.001)a
    T = answer

    is this correct?
  2. jcsd
  3. Nov 30, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What you need to compute is the angular acceleration ([itex]\alpha[/itex]). Just like linear acceleration is defined as change in velocity over time, angular acceleration is change in angular velocity over time. What's the change in angular velocity? (Convert rpm to rad/s.)
  4. Nov 30, 2007 #3


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    Homework Helper

    Use kinematics w = wo + alpha*t. Here wo = zero and w = 2*pi*1800/60. From these values find alpha and then torque.
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