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Homework Help: Yo-Yo problem

  1. Dec 7, 2013 #1
    1. The problem statement, all variables and given/known data
    A yoyo of mass m=2 kg and moment of inertia ICM=0.0625 kg m2 consists of two solid disks of radius R=0.25 m, connected by a central spindle of radius r=0.1875 m and negligible mass. A light string is coiled around the central spindle. The yoyo is placed upright on a flat rough surface and the string is pulled with a horizontal force F=28 N, and the yoyo rolls without slipping.
    The force is applied at the top of the spindle; so above the center of mass.

    Find the acceleration of the yo-yo.
    Find the frictional force
    2. Relevant equations

    3. The attempt at a solution

    I started taking moments about the centre of mass, which came to R*fr - r*F = Torque
    Then equated this with the Inertia and alpha.
    R*fr - r*F = I*alpha
    Then changed alpha to a/R.
    So ended up with a = ((R*fr - r*F)*R)/I
    I then use the F-fr=ma equation to solve for both fr and a.
    The answers i get are incorrect.
  2. jcsd
  3. Dec 8, 2013 #2


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    Homework Helper

    Are you sure the torque of the friction acts against the torque of the applied force???

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