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Homework Help: Finding Kinetic Energy of Person Spinning on Chair w/ Weights

  1. Oct 11, 2007 #1
    How do you find the Kinetic Energy of a person spinning on a chair with 2 equal weights on each arm?

    A specific problem asks what the change in KE is if he moves his originally extended arms inward.

    I tried doing (1/2)(I)(wf)^2 - (1/2)(I)(wi)^2 where wi is initial angular velocity and wf is final angular velocity, but its wrong.

    Any help?
  2. jcsd
  3. Oct 11, 2007 #2


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    Homework Helper

    What numbers/variables do they give?
  4. Oct 11, 2007 #3
    Ahh, sorry, I suppose I should have included this information in the main thread. My bad.

    Well I is the same for initial and final, which is 8. Therefore,

    Initial I = 8
    Final I = 8
    Initial w = .7
    Final w = 1.15

    So I did:

    (1/2)(I)(wf)^2 - (1/2)(I)(wi)^2
    = (1/2)(8)(1.15)^2 - (1/2)(8)(.7)^2
    = 5.31118 - 1.96
    = 3.35118116 j

    ...but apparently it's wrong.
  5. Oct 11, 2007 #4


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    I can't be the same initial and final... if he brings his arms in the moment of inertia changes. post the question exacty as it is stated...
  6. Oct 11, 2007 #5
    Alright. But just fyi, I know it cant be the same I because the r changes, but the problem says to just assume its the same. I used the same I for initial and final to get the wf, and it was correct, so i know I is same initial and final. Anyways, heres the problem:

    A student sits on a rotating stool holding two
    3 kg objects. When his arms are extended
    horizontally, the objects are 1 m from the axis
    of rotation, and he rotates with angular speed
    of 0.7 rad/sec. The moment of inertia of the
    student plus the stool is 8 kgm^2 and is assumed
    to be constant. The student then pulls
    the objects horizontally to a radius 0.29 m
    from the rotation axis.

    a) Calculate the final angular speed of the
    student. Answer in units of rad/s.
    (The answer to this part is 1.15232 rad/s)

    b) Calculate the change in kinetic energy of the
    system. Answer in units of J.
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