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Homework Help: Energy and Rotational Motion

  1. May 6, 2013 #1
    1. The problem statement, all variables and given/known data
    We performed a small experiment in class which had us attach a mass to a string which hung on a pulley which led to a rotating object.
    We were then told to write down a conservation of energy equation stating that the initial energy is equal to the final energy. We were asked to include kinetic energy (rotational, translational), potential energy, and energy lost due to friction which we found with Tf=f*r.

    2. Relevant equations

    3. The attempt at a solution

    This is what I have setup so far.

    Ui +Kroti +Ktransi=Uf+Krotf +Ktransf +Wf

    mgd=.5Iw^2 + .5mv^2 -f

    I'm still a little confused about how energy is conserved if we're losing energy due to friction?

    After that they ask me to replace every instance of w with v/r and v with 2d/t and try to get the following I=(mr^2)((gt^2/2d)-1-(t^2/(2md))f)

    What am I doing wrong here?
  2. jcsd
  3. May 6, 2013 #2


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    f is the force of friction? If so, there are two things wrong with the -f term.
  4. May 6, 2013 #3
    how in the world are you equating torque with energy
  5. May 6, 2013 #4
    I suppose I should have did f*Δd? I'm not sure what to do here.
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