Re: Cause and Effect

[Leigh Palmer <palmer@SFU.CA>]

At 2:01 PM -0400 10/24/00, Robert A Cohen wrote:
> On Mon, 23 Oct 2000, Leigh Palmer wrote:
>
> >  >Let me be a little clearer.  Might our use of the word "cause" be
> >  >inadvertently reinforcing this misconception by implying a
> >  >non-simultaneity between force and acceleration?
> >
> >  Do you believe that a cause must precede an effect in time? I would
> >  offer force and acceleration as a Gegenbeispiel. What I find
> >  difficult to understand is why anyone would doubt the causal relation
> >  between force and acceleration. Does anyone here seriously doubt it?
>
> Leigh,
>
> This has nothing to do with what I believe regarding the meaning of the
> word "cause".  For what it is worth, I repeat what I have written before:
> I personally use the word "cause" in the same way you do (although I think
> it is only for interpretation).  My point is that my students are more
> naive than you and may mistakenly interpret "cause" to support their
> belief that accelerations come after the force is applied.

Careful! You're reifying. Acceleration is a formal intellectual
construct; it is not a real thing. When we discussed the acceleration
of an object which is initially deformed by an impact we had to
define a center of mass which varied in time to make F = ma work.
Acceleration is simultaneous with force only because you define it
to be so, and you have to be pretty careful to maintain that model
quantitatively. If you think about center of mass as being a concrete
atom in, say, my wall example, then acceleration of that atom does
occur at a later time than the causal impact. Perhaps the concrete
example will make the student feel better about this description.

> This is a misconception, after all.  It is not correct.  However, a large
> number of our students think this way.  At least it seems to me (does the
> physics education research have anything to say on this?).
>
> I am just wondering if we may be inadvertently reinforcing this
> misconception.  Maybe I should use a different header?

I understand the problem; I just see no reason one should not simply
tell students that a cause need not precede an effect, but it *may*
do so. An effect cannot precede a cause. It's the same difference we
saw in describing the static frictional force, where a less than or
equal sign was used instead of a less than sign (or an equal sign).
Do we agonize over the difference? No! We do not claim to reveal
deep truths in physics; we merely describe Nature in the best way we
can. I claim that the best description I can make includes ascribing
causal status to something, perhaps fundamentally undefinable, which
I call "force", and I have no difficulty defining it operationally.
It has intuitive appeal, I maintain, and I believe the issue of
causality would not arise in the naive student's intuition. If you
have a JD in your class, acknowledge that philosophers worry about
such things, but physicists have pretty much figured it out, and the
explanation they give will stand up against a philosopher's in a
court of law. (I guess that means the lawyers will go along with it,
too.)

Leigh