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Conceptual Question on Angular Momentum.

  • #1

Homework Statement



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


Homework Equations



[/B]Conservation of Angular Momentum

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??
 

Answers and Replies

  • #2
963
213
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
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
 
  • #3
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
Ok. Thanks! :smile: so if the mass is extremely close and external torque=0, then angular velocity will reach very high.
 
  • #4
963
213
Ok. Thanks! :smile: so if the mass is extremely close and external torque=0, then angular velocity will reach very high.
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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