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Old Sep 1st 2012, 09:54 PM   #1
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The concept of conservation of angular momentum

I always read that conservation of angular momentum is with respect to an origin of our choice, so if we want to compare the angular momentum of two situations, we have to calculate the angular momentum in these situations with respect to the same origin. However - I've seen in some questions that there's conservation of angular momentum, but the calculation of it in each state is with respect to the center of mass. Now comes the twist - between these two states the center of mass itself has changed it's position in space (like with a rod that spins as it's making translation movement as well - in t=0 the rod can be at the left side of a room and in t=t' it can be at the right side of the room, it's clear that the center of mass of the rod is not at the same position in space).

I'd like to know what is the mathmatical justification for this comparison even though the origin between the two states is not at the same location.
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Old Sep 2nd 2012, 08:26 AM   #2
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The justification is that the two components of momentum - linear and angular - are vectors that can be added independently. The angular momentum components is most easily calculated with respect to the instantaneous position of the cm, which as you note may change as the object moves laterally. The first few pages of these notes may help:

http://ocw.mit.edu/courses/physics/8...rsenotes26.pdf

Also see the description of angular momentum here: http://en.wikipedia.org/wiki/Rigid_body_dynamics
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