1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    nevermind...its K trans + K rot = 160 + 80 = 240
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook