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

• From: "John S. Denker" <jsd@AV8N.COM>
• Date: Mon, 14 Mar 2005 22:48:25 -0500

On 03/14/05 21:57, Daniel Crowe wrote:
Just a minor quibble:

A pair of (equal and opposite) forces separated by a lever arm is a
couple. There is a nonzero moment (torque) associated with any single
force if its line of action doesn't pass through the origin.

Yes, AFAICT it's minor ... but I'd like to understand
it anyway, and I'm not yet succeeding.

By the third law of motion, there is no such thing as a
"single force". So isn't there pretty much a one-to-one
mapping between torques and couples?

I habitually consider torque synonymous with moment. It's
been years since I heard, saw, or used the term couple,
and I'm not sure if it refers to an important concept.

Perhaps it's one of those things like the "zero" of electrical
potential, which has no observable physical consequences,
so each person can choose it differently.

The choice of origin about which to measure angular
momentum is similarly inconsequential.

At the end of the day, the physically-observable thing
is transfer of angular momentum. Either A transfers
angular momentum to B or it doesn't. I always thought
torque captured that idea, and moment was the same as
torque.

Force is to torque as
momentum is to angular momentum.

If couple means something else, something worth knowing,

Maybe I'm missing something because the last 99 calculations
I did were in the CoM frame. Or ?????

====================

As I mentioned the other day in another context, whenever
I am the slightest bit confused about force (or any
analogous concept) I run home to the comfort of momentum
(or the analogous concept). I have a pretty robust
gut-level understanding of what it means to have local
conservation of something (momentum, angular momentum,
or whatever) in terms of continuity of world-lines.
http://www.av8n.com/physics/conservative-flow.htm
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