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Homework Help: Rotational Kinematics-YoYo Dropping from Ceiling

  1. Sep 13, 2008 #1
    1. The problem statement, all variables and given/known data
    Here's the problem in a JPG, with the image:

    http://docs.google.com/Doc?id=d277r7r_50c9trfbdr [Broken]

    (the 25 points is b/c it's a practice test)

    2. Relevant equations

    3. The attempt at a solution

    A)KE=0, PE=0 (defined to be at cylinder's initial position)


    C)Final KE=.5*M*(2gh)^2 (To get this, I used the v^2=vi^2 + 2aX equation, plugged it in for v^2 in KE=.5mv^2)

    However, I don't think my equations are right because if my KE was that, there would have to be extra energy put into the system for the yo-yo to rotate at all, thus there would be no rotational kinetic energy, which doesn't seem to jive with the original provlem. Where am I going wrong?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 13, 2008 #2

    Shooting Star

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    You are quite correct about you last eqn.

    The final KE = ½Mv² + ½Iω²., where ω is related to v. This should take take of your problem.
  4. Sep 13, 2008 #3
    Okay, so I know that I=(MR^2)/2, but you said that w is related to v. I don't see the relation?
  5. Sep 13, 2008 #4

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    Without giving you a direct answer, think of the angular displacement of any point on the disk and its relationship to the distance travelled by the centre. What is the relationship between the v and ω of a wheel rolling without sliding? I am sure you can take it from here.
  6. Sep 13, 2008 #5
    Hmm. Okay. So, if there's rolling w/out slipping, then v=wR. I then plugged that into the fact that -Mgh=½Mv² + ½Iω², and plugged into the w (that w=v/R). I did a bunch of algebra, and got that v=sqrt(-4gh/3), which works out to be a positive # b/c h is defined to be a negative distance fallen. Is that the right answer for the velocity of the center of mass?
  7. Sep 13, 2008 #6

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    Looks good, but I fail to see where the "lot" of algebra came in.

    [-Mgh = ½Mv² + ½Iω² = (3/4)Mv², by putting ω=v/R , cannot be counted as a lot of algebra...:cool:]

    However, looking at the original question, you are supposed to give the KE-rot in terms of R, M and v. That would just be (1/4)Mv².

    I think that covers everything. Good job.
  8. Sep 13, 2008 #7
    Sorry, I'm kind of lazy when it comes to simplification, so that's why I thought it was a 'lot' of algebra :). Thank you very, very much for your help.
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