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Re: [Phys-l] non-conservative --> non-grady ???



1. That the work done around a closed path is zero;
2. That the force is time independent and derivable from a potential.

Reggards,
Jack

On Thu, 21 Feb 2008, Alfredo Louro wrote:

On Thu, Feb 21, 2008 at 10:04 PM, Jack Uretsky <jlu@hep.anl.gov> wrote:
Hi all-
Since the two statements are mathematically equivalent, as far as
I can see, why is either one to be preferred?
Regards,
Jack


Well, I thought earlier I would conclude my correspondence on this
particular thread, but I can't help asking which two statements you
find mathematically equivalent?

Alfredo


On Thu, 21 Feb 2008, John Denker wrote:

> On 02/20/2008 09:57 AM, Alfredo Louro wrote:
>> "Piecewise time-independent" is not time-independent at all. And
>> whether a force is conservative cannot depend on what the particle is
>> doing. One way of defining a conservative force is to say the work
>> done by it around a closed path is zero. For any closed path. At all
>> times.
>
>
> Do we really want to define "conservative force" that way?
>
> I called attention to a situation
> http://www.av8n.com/physics/img48/accelerator.png
> where it was uniformly true that the field /applied to/ the
> system was
> a) independent of time, and
> b) the gradient of some potential.
>
> This is a statement about the field /applied to the system/
> at the time and place where the system happens to be.
>
>
> Do we really want to insist that the field be unchanging at
> all other times and all other places as well? That seems
> kinda strict. Forsooth, it guarantees that the suggested
> definition is vacuous. That is, there cannot be any field
> that satisfies the terms of this definition, because surely
> there is a non-constant field somewhere in the universe.
>
>
> This example seems to reinforce the point I was making at the
> beginning of this thread: The whole business of "non-conservative"
> force field is just begging to be misunderstood.
>
> To summarize:
> -- If you mean grady, say grady. Calling a non-grady field
> "non-conservative" is asking for trouble.
> -- Both grady and ungrady force-fields uphold conservation of
> energy. No exceptions have ever been observed.
> -- Conservation is not the same as constancy. Local conservation
> of XX means that XX is constant /except/ insofar as it flows
> across the boundary.
> -- For an isolated system, conservation would be the same as
> constancy, but that's an almost trivial subset of physics.
> Physics relies on /local/ conservation laws that can be
> applied to non-isolated systems and subsystems.
>
>
> http://www.av8n.com/physics/conservative-flow.htm
> http://www.av8n.com/physics/non-grady.htm
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
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>

--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley



_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley