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# [Phys-L] Re: conservation of angular momentum question

On 3/14/05 5:09 PM, "Daniel S. Price" <dprice@JEFFCO.K12.CO.US> wrote:

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).
...

Howdy,

Since the axis are separate an external torque is applied on those axis. Try
to imagine what would happen if the non-rotating cylinder would be (gently)
dropped on the rotating cylinder; it would fly off and the rotating cylinder
would tend to fly off in the opposite direction. It takes and external
couple to keep this from happening.

Good Luck,

Herb Schulz
(herbs@wideopenwest.com)
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