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Homework Help: Rotating Ball

  1. Apr 2, 2008 #1
    A uniform, solid sphere of mass 3.2 kg and diameter 63 cm is rotating about its diameter at 230 rev/min. What is the kinetic energy of rotation?

    230rev/min= 24.1 rad/s

    I know that KE=1/2Iw^2 and interia of a solid sphere is I=2/5MR^2

    I=(2/5)(3.2)(.63)^2
    =.508032

    KE=(1/2)(.508032)(24.1)^2
    =147.5 J

    For some reason my answer is wrong. Am I missing something? What am I doing wrong?
     
  2. jcsd
  3. Apr 2, 2008 #2

    Dale

    Staff: Mentor

    Yup, just a little mistake.

    PS Welcome to the PF!
     
  4. Apr 2, 2008 #3
    63 cm

    But doesn't 63cm need to be converted to meters making it .63 meters?
     
  5. Apr 2, 2008 #4

    Dale

    Staff: Mentor

    It needs to be converted to a radius too!
     
  6. Apr 2, 2008 #5
    Thanks

    I see now! LOL! My bad. Very dumb mistake! Thanks!
     
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