Re: Cause and Effect

[John Denker <jsd@MONMOUTH.COM>]

At 01:25 PM 10/22/00 -0400, Ludwik Kowalski wrote:
> Consider a ball suspended by a thread. Two forces act on
> it at rest: W=mg and T. The net force is zero, there is no
> acceleration. Now release the thread. The force T becomes
> zero and the ball starts accelerating. The force W=mg existed
> before, but  now it becomes Fnet. That is why "force is a
> cause of a", not the other way around.

There are so many fallacies here one hardly knows where to begin.

1) This fails to distinguish
 -- the concept of "calculated from"
versus
 -- the concept of "caused by"

Certainly there are cases where it is convenient to calculate the force
first, and then calculate the acceleration -- but this proves nothing about
what _causes_ what.

In this thread examples multiple examples have already been given of things
that are most conveniently calculated in one order or another.

2) It has been known for thousands of years that "post hoc ergo propter
hoc" is a fallacious argument.  Saying "the force T becomes zero and the
ball starts accelerating" proves nothing about what causes what.

For that matter, the change in acceleration does not occur "post" the
change in force, but rather at the same time, so we have failed even to
establish the premises for this fallacious argument.  That is, we have
fallacy squared.

3) This fails to distinguish between the particular case and the general
case.  The present example, if the other fallacies could be cleaned up,
could at best support the proposition that "force _sometimes_ causes
acceleration, not the other way around".  It comes nowhere near proving
that "force always causes acceleration, never the other way around."

4) Apparently part of the argument is the unstated notion that Ludwik
_intended_ to change the force by snipping the string.  But the laws of
physics say that you can't change the force without changing the
acceleration, so what does it matter what anyone intended?  The laws of
motion don't care what anyone intended.

As I said before, you can draw an irreversible arc from intention to force:
  intention ==> force
in this particular case, but that proves nothing about the relationship of
force and acceleration in this case, let alone the general case.

Go ahead and assert whatever unmeasurable assertions you want -- but don't
pretend your dogma is based on the laws of motion, and don't be surprised
if your neighbor chooses a different set of assertions.

====================================

There remains the pedagogical / psychological question of why people would
cling to the idea that "accelerations are caused by forces and not vice
versa" in the utter absence of evidence.

One possibility might be lack of broad experience, compounded by the force
of habit.  If the only problems you've ever done are homework problems,
posed by somebody who _chose_ to make force the independent variable and
acceleration the dependent variable, then you might well have the habit of
thinking in those terms.  A slightly broader set of exercises will change
the story quite a bit:
 -- calculate the force on the bearings of an unbalanced wheel
 -- calculate the pressure at every point in a uniformly-rotating
    bucket of water
 -- calculate the force on the wing of an airplane during a
    standard-rate turn (3 degrees per second)
These are all simple problems.  I would hope any freshman-physics class
could learn to handle them.  And I would hope they would handle them by
using the statement of the problem to determine the acceleration, and
thence calculating the force.

Slightly broader exposure to theoretical physics (e.g. time-reversal
symmetry) would change the story quite a bit, too.

==========

More generally, it appears that people -- all people -- have an innate
fondness for cause-and-effect stories.  It's one of the few things that are
cultural universals -- possibly just as universal as clothes.  For untold
thousands of years, people have turned to astrologers and witch doctors to
find out "causes" for the vicissitudes of life.  People want to know
"why".  Often they aren't too fastidious about checking the correctness of
the answer, but they demand an answer.
 -- The witch doctor might say the hunt failed because you wore your
amulet on the wrong arm today.  People are happy because they now know the
"cause" of what happened.
 -- Sometimes students want to know why a wing produces lift.  The
instructor says "curved on top and flat on the bottom" and the student goes
away happy -- even though that "explanation" is inconsistent with many
obvious facts about the situation.
 -- Sometimes it's expedient to say "forces cause accelerations and not
vice versa" to shut somebody up, even if that has no basis in the laws of
motion.