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Re: Conservation Laws



At 09:41 AM 3/13/00 -0600, E.C. Muehleisen wrote:
In the Candide-like world of physics storerooms there are rigid weightless
rods, point masses, frictionless surfaces and totally inelastic collisions
as well as other oddities.

Some of those things are not real, but are merely shorthand expressions for
the limiting case of real objects and real processes.

My problem is with the mutual exclusivity of the
conservation of momentum and conservation of energy laws.

The laws are not mutually exclusive. They are both true, always.

Consider a moving
rail car and a stationary rail car on the same frictionless, horizontal
rails.

So far so good.

The moving car strikes and couples to the stationary one in a
totally ineleastic fashion.

OK. This was discussed a while back on this list.

No dissipative forces are about.

That's the fallacy. You can't have an inelastic collision without
dissipation.

(Sometimes people devise bizarre processes that mimic inelastic collisions
and/or mimic dissipation, but we don't need to go there now.)

'Tis indeed a closed, conservative system.

Possibly yes, if you count the thermal degrees of freedom, and if no heat
(or sound or ...) is transferred to the environment.

Yet the final speed of the coupled pair is not
the same using the two laws.

But you always use the two laws. Both of them.

If indeed this is the best of all possible
worlds, why do not the speeds from the two calculations agree?

What two calculations? I only know of one calculation.