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Homework Help: Circular Platform

  1. Nov 10, 2007 #1
    A horizontal circular platform (M = 92.1 kg, r = 3.49 m) rotates about a frictionless vertical axle. A 73.3 kg student walks slowly from the rim of the platform toward the center. The angular velocity of the system is 3.5 rad/s when the student is at the rim. Find the angular velocity of the system when the student is 1.61 m from the center.

    So I have these calculations:

    I=92.1*3.49^2+73.3*1.61^2= 1311.79

    Rotational Kinetic Energy:

    I haven't been able to get the right answer of 6.78 rad/s. I'm reviewing for a test by reworking my homework, but I'm not sure how I got that answer.
  2. jcsd
  3. Nov 10, 2007 #2


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    Homework Helper

    Use conservation of angular momentum.

    [tex] L_i = L_f [/tex]

    so [tex] I_i \omega_i = I_f \omega_f [/tex]
  4. Nov 10, 2007 #3
    I tried that and still got a wrong answer:5.375
  5. Nov 10, 2007 #4


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    Homework Helper

    OK I just noticed you are using the same moment of inertia expression for both "objects". You need to use this

    for the wheel [tex] I = \frac{1}{2}MR^2 [/tex]

    for the student, take as a point mass so [tex] I = mr^2 [/tex]
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