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Homework Help: Rotation only of yo-yo

  1. Mar 13, 2010 #1
    1. The problem statement, all variables and given/known data

    Realize I'm asking a lot of question today, but I'm working on a huge physics project and I'm not asking for answers, just some guidance where I'm stuck

    A certain yo-yo can be modeled as a uniform cylindrical disk with mass M and radius R and a lightweight hub of radius ½R. A light string is wrapped around the hub.

    imagine that that you pull upward on the string such that the yo-yo remains in the same place. Find the angular velocity of the yo-yo when you have pulled the string upward a distance of L.

    2. Relevant equations

    Torque = r x F
    Work = change in kinetic energy
    torque = I x angular acceleration
    only rotation = change in (1/2 I w^2)

    3. The attempt at a solution

    w2=w02 +2(alpha=angular acceleration) * (theta -theta0) -->

    torque = I (W2/2(theta-theta0) -->

    torque * change in theta = 1/2*I*w2

    Now I just need to figure out how to find theta or what to do with it? Maybe get it in terms of L? but how? Does torque * change in theta = the change in kinetic energy?
  2. jcsd
  3. Mar 13, 2010 #2


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    Hint: You also need F=ma.
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