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Homework Help: Yo-yo problem - rotational motion

  1. Dec 7, 2009 #1
    1. The problem statement, all variables and given/known data

    consider a yo-yo with R = 3.3 cm, r = 0.3 cm, and mass m = 84 g.

    What is the speed of the yo-yo after it has fallen a distance h = 1.20 m?

    What is the tension in the string while the yo-yo falls?

    What is the magnitude of the acceleration of the centre of mass as the yo-yo falls?

    2. Relevant equations

    L = Iw
    I = 0.5MR^2
    KEr = 0.5Iw^2
    Tnet = Ia

    3. The attempt at a solution

    change h into rotational displacement, not sure where to go or if thats even right.

    can anyone give me direction????
    Last edited: Dec 7, 2009
  2. jcsd
  3. Dec 7, 2009 #2
    would you solve this using conversation of energy?
  4. Dec 7, 2009 #3


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    Homework Helper

    Yes, I would use energy for the first part. You have both rotational and translational energy and they are related by v = rω. The r = 0.3 cm must be the radius of the center "axel" that the string unwraps from.

    If you don't put in the h = 1.2 too soon, you'll have the speed as a function of h. Perhaps you can differentiate that with respect to time and get something you can use to find the acceleration.
  5. Dec 7, 2009 #4
    what would the moment of inertia be? 0.5mr^2, would you use the large or small radius?

    would the height be 1.2 m? for PE
  6. Dec 7, 2009 #5


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    Yes, the large radius for moment of inertia.
    Yes, the initial height is 1.2 m.
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