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Homework Help: Formula for total KE of disk rolling across table 8 m/s?

  1. Nov 29, 2009 #1
    1. The problem statement, all variables and given/known data

    A solid disk (mass = 5 kg, R=0.9 m) is rolling across a table with a translational speed of 8 m/s.

    What is the total KE of the disk?

    2. Relevant equations

    K = the sum of (1/2 x mass initial x velocity initial^2)

    (in my textbook, there are other derivations of this formula using the variable angular velocity instead of velocity, but they yield the same answer)

    3. The attempt at a solution

    KE = 1/2 x 5 kg x 8^2 = 1/2 x 5 kg x 64 = 160 (this was incorrect)

    I also plugged in 80 just for the heck of it, and this was incorrect as well.

    In the previous question, I was asked to find rotational KE, and I made the mistake of forgetting the coefficient (1/2) in calculating the moment of inertia. However, this KE formula does not include the moment of inertia, so this does not seem to be a problem here.
     
    Last edited: Nov 29, 2009
  2. jcsd
  3. Nov 29, 2009 #2
    ugh

    nevermind...its K trans + K rot = 160 + 80 = 240
     
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