Re: Causation in Physics: F=ma

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

Just a few comments below:
On Fri, 14 Nov 2003, John S. Denker wrote:

> But this does not prove that F causes ma, for multiple
> reasons:
>
>   1) Examples, even innumerable examples, do not prove
> the general case, especially when there are counterexamples.
> For instance, if I tell you the mass, radius, and
> rotation-rate of a centrifuge, you can calculate the
> acceleration and thence calculate the force.

This is particularly troubling since in any inductive or reductive
activity like science, you never can prove in the sense that you can in a
geometry that is deductive.  The best you can do is decide the there are
enough consistences to believe that something is correct, or more
accurately it is tentatively correct.  So you never prove the general
case, no matter what.  And if you have a counterexample, then there is
nothing to prove.  Perhaps, John, you want to rephrase what you wrote?

> >
2) As a point of fundamental logic,
the cause of our > *knowledge* of ma is not the cause of ma itself.

It seems to me that in the Newtonian agenda, at least, we observe that
continued constant motion is the natural state of affairs, and thus
changes from that motion need explanation and hence cause.  The name we
give to that is force.  If we want to quantify that we look to ma, put it
seems to me that the establishment of force as cause is prior,
conceptually, to the quantifacation.

> >   3) Cause should precede effect.
The force F does > not precede the acceleration a=F/m ... they must
> both happen at the same time.

If we think of macroscopic objects, then there is a time delay between
the application of the force and the response of the object, since the
influence propogates at about the speed of sound.

so it seems to me appropriate to think of "force" as cause of change in
velocity.

cheers

joe


Joseph J. Bellina, Jr.               574-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556