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Re: Conferring its nature (was "are normal ...?")

At 04:16 PM 6/30/01 -0400, Ludwik Kowalski wrote:

Consider two positively charged objects which attract each
other gravitationally. An equilibrium will be established
when attractive and repulsive forces are equal.

Really? How is it that two forces that are initially unequal later become
equal, when both follow the 1/r^2 law?

I think that equilibrium requires something more than electrostatics and
gravitation.

In this case we can say that one mass rests on another and that the
reaction force (really an electric force) is conservative.

The reaction is not just an electric force. You need something more than
electrostatics to make a workable theory of chemistry and materials
science. Something like an exclusion principle helps a lot.

Is the conservative nature of the electric force "conferred to"
the normal force? I would think so.

The same is true for a common case in which normal
forces are identified with perfectly elastic forces.

Enough guessing and hand-waving already. Let's not talk about vague
notions of "conferring" this or that. Let's talk about how certain
physical facts can be derived from previously-known physical facts.

One can easily show that a force of constraint (which by definition is
normal to the motion along the constraint) does no work at all _provided_
the constraint does not itself move. The notion of "no work at all" is
narrower than (and certainly implies) the notion of "conservative".

There is a really simple answer to the original question:
A moving constraint is not, in general, conservative.

I don't see any reason to make it more complicated than that. Raising the
ramp = moving the constraint. There was never any reason to expect that a
moving constraint would be conservative -- except for the all-too-human
pseudo-reason that people seem to remember the result "constraint ==>
conservative" without bothering to remember that it is restricted to
non-moving constraints.