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Re: CAUSATION IN PHYSICS



Gee John, maybe you're just too damn smart to waste your time with the
people on this list. ;-)

Maybe you should also listen to what most people are saying:

1) Accelerations are caused by forces.

2) Forces are not caused by accelerations.

In one sense I agree with you, F = ma is an equivalence, not a causation
relationship. However, I don't think many (if any) have been saying F
causes ma. I personally don't like the relationship expressed this way. I
much prefer a = F/m as a more operational relationship. I would express it
as: "In order to accelerate an object you need to apply a (net) force to the
object and then the resulting acceleration will be inversely proportional to
the mass of that object."

If you can't figure out why 'bring up causation in an introductory mechanics
course' you either haven't taught one for a long time and/or haven't paid
any attention to the Physics Education Research people. I can't imagine
teaching such a course without dealing with cause and effect or being so
dogmatic and 'mathematically pure' that I could hope to do so.

Rick

**************************************************
Richard W. Tarara
Associate Professor of Physics
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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----- Original Message -----
From: "John Denker" <jsd@MONMOUTH.COM>


Hmmm, when people have a strong "preference" for unfacts, and an explicit
disinterest in the facts, I'm wasting my time explaining the facts.

At 09:24 PM 10/14/00 -0400, Ludwik Kowalski wrote:

In my opinion natural laws are cause and effect relations.
...
I am comfortable with F being a cause of acceleration. What is wrong with
this?

Here's what's wrong: we have a conflict between
-- a "comfort zone" rooted in opinion and tradition, and
-- facts and formal definitions.

There are certain properties that an "equivalence" relation needs to
have. There is a formal definition that captures these properties. Most
people don't know the definition.

There are certain properties that a "cause and effect" relation needs to
have. There is a formal definition that captures these properties. Most
people don't know the definition.

I hope most people on this list, if they thought about it for a moment,
would agree with the observation that "equality" is symmetric while
"causation" is nonsymmetric.

I feel like I'm arguing against astrology. For thousands of years people
have wanted to believe there is a cause-and-effect relationship between
planetary motions and human affairs. Even today in Japan, people are
careful to schedule weddings etc. on astrologically "auspicious" days.
But
let's not pretend there is any physics in it.

Just because people _want_ to find a cause-and-effect relationship in F=ma
doesn't mean one really exists.

At 04:12 PM 10/14/00 -0500, John M. Clement wrote:
My comments are pedagogical ... high school ...

The thing that confuses me most about this discussion is this: Why bring
up "causation" at all in an introductory mechanics course?

1) I would be very surprised if significant numbers of high-schoolers
would, without prompting, formulate the question "Does F cause ma or vice
versa?". If anybody has data on the subject, please let us know.

2) I would be even more surprised if any student who asked such a question
would refuse to accept the correct answer, namely "you can't have F
without
ma, and you can't have ma without F; this relationship is called
equivalence, not causation".

3) Causation should be discussed a little bit in connection with
thermodynamics (the arrow of time) and a little bit in connection with
special relativity. Serious discussion should wait until the students
have
learned enough probability to understand concepts such as "conditional
independence" and "sufficient statistic".

=================
Since people are stating their preferences, here is where I come down:

*) I'm 100% in favor of making appropriate approximations. I have never
advocated miring students in unnecessary detail.

*) But I'm opposed to saying things that are diametrically wrong when it
is
completely unnecessary.

Saying
"F causes ma, therefore ..."
is almost as silly as saying
"The autumnal equinox is in Aquarius, therefore ..."