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Homework Help: Angular kinetic energy

  1. Mar 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A spool of thin wire (with inner radius R=0.50m outer radius R=o.65m, and a moment of inertia I_cm=0.8957 kg*m^2) pivots on a shaft. The wire is pulled down by a mass M=1.5 kg. After falling a distance D=0.51m, starting from rest, the mass has a speed of v=64.5 cm/s. Calculate the energy lost to friction during that time.

    2. Relevant equations

    3. The attempt at a solution
    I have made two attempted solutions for this problem. The first was simple. I neglected the spool all together. Then I solved for the potential energy of the mass pulling down and the kinetic energy of this mass, and found the difference... that didn't work.
    My second attempt was to calculate the kinetic energy of the spool with KE=.5Iw^2 and compare it to the Kinetic energy of the mass pulling down but to solve for the spools kinetic energy i need the moment of inertia about the axis of rotation, which means i need the mass of the spool, which i don't have... any suggestions?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 20, 2008 #2


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    Gold Member

    I think you have to assume that the wire that unwinds does not change the MoI of the spool and wire. So your second method should work if you calculate w (omega) correctly. You have the outer dimension of the spool so that is straightforward.
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