Re: F=ma

[Leigh Palmer <palmer@SFU.CA>]

At 10:03 -0800 11/7/99, Joel Rauber wrote:
> > > Or just as dQ + DW "causes" a dE in the First Law.
> >
> > I don't view either of those as causal relations. That's just
> > my personal preference now; I probably did at one time.
>
> I was going to continue the DQ, DW discussion with the following observation
> that early writers often write the first law as:
>
> DQ = dU +DW  (see Martin Bailyn's "A survey of Thermodynamics", pg 63)

I don't have this book. That is the engineers' way of writing the
first law. It makes sense from the viewpoint that work is what one
gets out of an engine.

> and I was going to express the idea that perhaps this is a better way to
> view matters, particularly with respect to how one should partition the
> energy inputs to the system of interest.

I think the reason I can no longer put a causal direction on the
first law is that the convention for ascribing a sign to dW (and,
for that matter, to dQ as well) is arbitrary. I think that the
causal view reifies energy, and I am uncomfortable with that. It
is not necessary to do so; the law of conservation of energy
(unlike the second law of thermodynamics) has no intrinsic
direction in time. Thus the question of causality should never
arise when the first law is invoked.

I recognize that what I've said above applies also to Newton's
second law when it is applied to forces of contact. There is a
possibility of a causal element when Newton's law is applied to
forces that act at a distance, however.

Anyway, this discussion has reminded me that F=ma is not Newton's
second law of motion; it is only part of that law, which Newton
stated in words explicitly recognizing the causal nature of the
interaction. I think now that the direction of the equation
probably doesn't matter as much as I had thought it did, since
several people in this group parse it actively while I parse it
passively. Perhaps the best teaching strategy is to note the
diversity in interpretation which is possible and to show the
students both ways, permitting each to choose the version which
is more comfortable.

Leigh