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Homework Help: Rotation energy

  1. Feb 22, 2005 #1
    The center of mass of a pitched base ball (radius=3.8cm) moves at 38m/s. The ball spins about an axis through its center of mass with an angular speed of 125 rad/s. Calculate the ratio of the rotational energy to the translational kinetic energy. Treat the ball as a uniform sphere.

    [tex]0.5Iw^2:0.5mv^2[/tex]
    [tex]I=mr^2=m(.038m)^2=m0.001444[/tex]
    [tex]0.5m0.001444(125rad/s)^2:0.5m(39m/s)^2[/tex]
    [tex]22.5625:1444[/tex]
    [tex]1:64[/tex]

    is this correct?
     
    Last edited: Feb 22, 2005
  2. jcsd
  3. Feb 22, 2005 #2
    The moment of inertia for a uniform sphere is as follows:

    [tex]I = \frac{2}{5}m\,r^{2}[/tex]
     
  4. Feb 22, 2005 #3
    thanks:

    [tex]0.5Iw^2:0.5mv^2[/tex]
    [tex]I=(2/5)mr^2=m(2/5)(.038m)^2=m5.776E-4[/tex]
    [tex]0.5m(5.776E-4)(125rad/s)^2:0.5m(39m/s)^2[/tex]
    [tex]9.025:1444[/tex]
    [tex]1:160[/tex]

    is this correct?
     
  5. Feb 22, 2005 #4
    Looks good to me..
     
  6. Feb 22, 2005 #5
    why is the kinetic energy so much larger than the rotational energy?
     
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