Finding Rotational Speed once the Moment of Inertia Increase

  • #1

Homework Statement


A trapeze artist performs an aerial maneuver. While in a tucked position, as shown in Figure A, she rotates about her center of mass at a rate of 6.03 rad/s. Her moment of inertia about this axis is 16.1 kg·m2. A short time later the aerialist is in the straight position, as shown in Figure B. If the moment of inertia about her center of mass in this position is now 33.1 kg·m2, what is her rotational speed?

Homework Equations


KE=.5*m*v^2 w(omega)= v/r

The Attempt at a Solution


I've been struggling with this course all year. This is my first physics course outside of college. My biggest problem is that my professor mainly does derivation in class. Once its time for homework were we are using values instead of variables I freeze up. Any help is really appreciated. I just need a nudge in the right directions to start this problem.
 

Answers and Replies

  • #2
SteamKing
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You need to pick a relevant equation which relates the moment of inertia of a body to its rotational speed. The equation you show in the OP is not that equation.

What have you learned about bodies which rotate about an axis? Do any of the equations in the following article look familiar?

http://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis
 
  • #3
L=I*w(omega), Since I have the omega and the moment of inertia I can find the angular momentum, but how do I relate it to the rotational speed?
 
  • #4
SteamKing
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L=I*w(omega), Since I have the omega and the moment of inertia I can find the angular momentum, but how do I relate it to the rotational speed?
You are given the initial rotational speed in the problem statement, before the MOI changes. After all what does omega represent?

Since no external forces of moments are acting on the trapeze artist, what can also be said about the angular momentum before and after the MOI changes?
 
  • #5
I got it! Momentum is conserved so by using L=Iw and the givens i found w(final). Thank you for your help!
 

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