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Homework Help: Rotational Energy

  1. Dec 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A dancer spinning at 72 rpm about an axis through her center with her arms outstretched. The distribution of mass in a human body is
    Head: 7%
    Arms 13%
    Trunk and legs: 80%
    Using your own measurements on your body calculate your
    a) moment of inertia about your spin
    b) rotational kinetic energy


    2. Relevant equations
    I decided to use two equations: the solid sphere and solid cylinder
    I=2/5 MR^2
    I=1/2 MR^2

    3. The attempt at a solution

    75 kg for weight, 10 cm for head, 60 cm for arms, 90 cm for trunk.

    .07*75=5.25
    .13*75=9.75
    .8*75=60

    Head: 2/5*(5.25)(5)^2=52.5
    Arm: 1/2*(9.75)*(30)^2=4387.5
    Trunk: 1/2(60)(45)^2=60750
    I add those up and got 65190


    I'm not sure if I did this right.
     
  2. jcsd
  3. Dec 3, 2008 #2
    Never mind, I saw what I did wrong!
     
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