The source of external torque is in the support of the cylinders, you
can choose a rotation axis for calculating torques so that it is zero
for one of the supports, but not the other, as the axis of the supports
are necessarily not coincident as in the other problem where you may
conserve angular momentum (same support for both cylinders).
No time for a lengthier reply at this moment.
Department of Physics - SDSU
| -----Original Message-----
| From: Forum for Physics Educators
| [mailto:PHYS-L@list1.ucc.nau.edu] On Behalf Of Daniel S. Price
| Sent: Monday, March 14, 2005 5:10 PM
| To: PHYS-L@LISTS.NAU.EDU
| Subject: conservation of angular momentum question
| I should be able to answer this, but I'm clearly missing
| something. It may be in my visualization, it may be in my
| interpretation, or I may just be dense. Regardless, I throw
| myself upon your mercies:
| A classic application of conservation of angular momentum,
| often cited in physics texts, is the joining of two cylinders
| rotating about a common axis. When the cylinders are allowed
| to meet "face-on", the angular momentum of the system is
| conserved. When cylinders rotating about parallel axes meet
| "edge-on", however, the angular momentum of the system may
| not be conserved.
| In the latter case, imagine two cylinders, one initially
| rotating and the other stationary. If they are gently
| brought into contact, frictional force between the cylinders
| acts to slow the original cylinder's rotation and induce
| rotation in the other cylinder. HRW (fifth edition), chapter
| 12, question 49, is an example of this situation, and in the
| problem the authors claim that angular momentum is not
| conserved. If they mean that the angular momentum of the
| system is not conserved, I do not see the source of the
| external torque (unless the frictional force is responsible,
| though it would seem to be internal to the system as it acts
| only between the cylinders).
| The former case is also discussed in HRW5, chapter 12,
| question 53; the coupling of the cylinders "face-on" is said
| to maintain angular momentum (though friction between the
| cylinders seems to be responsible for their eventual rotation
| as a unit).
| Why are the two situations different in how conservation of
| angular momentum is applied? I seem to be missing something
| which I feel is very important.
| --Daniel Price
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