Re: F=ma

[Leigh Palmer <palmer@SFU.CA>]

At 18:01 -0800 11/5/99, Jim Green wrote:

> At 07:02 AM 05-11-99 -0800, Leigh wrote:
> > I've always looked at Newton's second law as being somehow incomplete.
> > It does not include what I consider to be a causal element. Forces do
> > cause accelerations, at least that is what my intuition tells me, or I
> > was successfully brainwashed a long time ago.
>
> I never disagree with Leigh -- I have not found a need to.  But sometimes I
> need help in understanding:
>
> Just why is it that a "causal element' seems to be  missing?

Newton's algebra doesn't admit of an asymmetrical use of "equals".

> Why should I not think in terms of a net force "causing" an acceleration,
> Fn=ma?

I think it is natural (and intuitive) to do so. I believe that
the formal statement with F on the left may make that causal
relation somewhat less evident. The other way seems - I've no
other word for it - more sensible. I feel an asymmetry which
is not formally acknowledged in algebra. It is not blessed by
canon, but if there is a less formidable barrier to
understanding a relation in its more sensible formulation, why
not do it that way?

I don't take it as manifest that what I propose is more
sensible; my proposal is based on emotion alone, and as I
pointed out before it may be colored by culture. That's what I
think I would like to know. I've made my preference clear; do
others in the group have the *feeling* I've expressed? Does
anyone feel the other, more conventional ordering is better?
It certainly has had a huge head start with all of us.

> Just like W "causes" dKE in the W/E theorem?
>
> 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.

> Or V=Ir -- potential "causes" current.

As I mentioned before, I prefer J = sigma E. Water does tend
to flow downhill, but I can build a conceptual model with
forces more intuitively than I can with potentials. I do both
now, but I'm fairly sophisticated; I think forces came easier.

> BUT not like E=mc^2 which seems to be an equivalence
> ie E _is_ mass -- not E "causes" mass

Right. This, too, is in the category of acausal relations.

> Maybe I am just saying when it was that I went to school.

I think you are right. In that case our feelings are cultural
to some degree.

> What have I missed??  I am starting to miss things all the time these days.
> Help me out here.

Gosh, Jim, I'm 64. I understand!

Leigh