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Conceptual Question on Angular Momentum.

  1. May 9, 2016 #1
    1. The problem statement, all variables and given/known data

    A Person sitting firmly over a rotating stool has his arms stretched. If he fold his arms, his angular momentum about the axis of rotation :
    A.) Increases
    B.) Decreases
    C.) Remains Unchanged
    D.) doubles

    2. Relevant equations

    Conservation of Angular Momentum

    3. The attempt at a solution

    Since External Torque =0
    Then final angular momentum = initial angular momentum
    => (C.)

    But I wanted to further ask that in the Equation L=I*(omega)
    since L has to be constant(if external torque = 0) then:
    If I increases then omega decreases to keep L constant
    or if omega increases then I decreases to keep constant

    Will folding his arms increase I( Moment of Inertia ) or decrease it??
  2. jcsd
  3. May 9, 2016 #2
    by folding his arms the new moment of inertia of the system will increase /decrease?
    how moment of inertia depends on mass distribution? if a mass is farther from axis of rotation its moment of inertia should be larger as I= m.r^2
    L= I . angular velocity therefore a decrease/increase in I should result in the angular velocity to increse/decrease
  4. May 9, 2016 #3
    Ok. Thanks! :smile: so if the mass is extremely close and external torque=0, then angular velocity will reach very high.
  5. May 9, 2016 #4
    the example is ice-skating sport -there the rotation speed of skaters is increased or decreased by them using their arms folded or spreading out.
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