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Homework Help: Rotation: Yo-Yo

  1. Nov 24, 2013 #1
    1. The problem statement, all variables and given/known data

    A Yo-Yo of mass M has an axle of radius b and a spool of radius R. Its moment of inertia can be taken to be MR[itex]^{2}[/itex]/2 and the thickness of the string can be neglected. The Yo-Yo is released from rest. The center of the Yo-Yo descends distance h before the string is fully unwound. Assuming it reverses direction with uniform spin velocity, find the average force on the string when the Yo Yo turns around.

    2. Relevant equations

    v=bα v = linear velocity of the yoyo. α= angular velocity of the yoyo.

    [itex]\frac{1}{2}[/itex]Mv[itex]^{2}[/itex] + [itex]\frac{1}{2}[/itex]Iω[itex]^{2}[/itex] = Mgh

    3. The attempt at a solution

    From the constraint equation and the energy conservation I calculated final ω. Now change in angular momentum is 2Iω. Now I can't find the average force from here.
  2. jcsd
  3. Nov 24, 2013 #2

    Simon Bridge

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    Homework Helper

    You are asked to assume a uniform spin velocity - since the rotation changes neither rate not direction (constant velocity) how does it change angular momentum?

    What changes direction when the yo-yo reaches the bottom of the string?

    You do have a problem with "average force" though ... what would this mean?
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