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Homework Help: Rotation Problem

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

    A hoop, rolling at a constant velocity of 4m/s, rolls up a hill .8 m tall on to flat surface .
    a) what percent of its initial total K.E. ( before rolling up the
    hill) is due to rotational K.E.
    b) how fast is it moving on top of hill.

    2. Relevant equations

    3. The attempt at a solution
    i am completely stumped, every equation i can find either requires a mass or a R or both. This problem looks like it should be easy thats why i feel so dumb for not knowing it.
  2. jcsd
  3. Dec 7, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Just call the mass 'm' and the radius 'R'. You don't need actual values to answer the questions.
  4. Dec 7, 2009 #3
    well my study guide gave me the answers a)50 % b) 2.86. and i dont understand at all how they got that.
  5. Dec 7, 2009 #4
    The plot deepens from that of the last problem--(man you guys are trying to bite off a lot at one time this w/e):


    KE(rotational)=1/2 I*w^2
    KE(translational)=1/2 m V^2

    now just pick an arbitrary R and m and put both equations in common terms; hint use w=vr and your other eqn I=mR^2. That should take care of probllem 1, but finish the other problem first. Mutitasking is good but not in physics!:smile:
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