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Homework Help: Stumped - work done by rotating mass

  1. Mar 14, 2005 #1
    stumped -- work done by rotating mass

    the question is http://home.earthlink.net/~urban-xrisis/clip_image002.jpg [Broken]

    I got the answers to questions (A) and (B) but I cant get (C)
    My work is shown http://home.earthlink.net/~urban-xrisis/clip001.jpg [Broken]

    the books gives me an answer of [tex]W=0.5mv_i^2(r_i^2/r^2-1)[/tex]

    any help on where I went wrong?
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Mar 15, 2005 #2

    Doc Al

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    Staff: Mentor

    You went wrong when you tried to find the work by multiplying the tension by [itex]r_i - r[/itex], which treats the tension as though it were constant. (It's not.) Instead, you must integrate to find the work done, since the tension is a function of r.
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