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Re: [Phys-l] F causes a



Al Bachman wrote:
Let me try to clarify my position.

1. I agree that Newton's Second Law does not show which is
cause and which effect.

OK.

> 3. If the subject is Newtonian mechanics, I don't see how one
> can avoid the name "Newton".

But if the subject is physics, one ought to be allowed to
entertain the hypothesis that Newton was
-- perhaps quoted out of context,
-- perhaps misquoted,
-- perhaps mistranslated, and/or
-- perhaps wrong.

2. In the context of teaching Newtonian mechanics, and in
particular, Newton's three laws, that Newton's FIRST Law
is interpreted by students as saying that forces cause accelerations.

As has been pointed out, the "authoritative" statement of
the first law is

Corpus omne perseverare in statu suo quiescendi vel movendi
uniformiter in directum, nisi quatenus a viribus impressis
cogitur statum illum mutare.

Reference: http://www.preces-latinae.org/thesaurus/Introductio/Principia.html

Misspelled versions are alarmingly common.

As I previously stated, the verb here is /cogitur/. That is
the present indicative passive of /cogo, coegi, coactus/ as
you can verify via
http://humanum.arts.cuhk.edu.hk/Lexis/Latin/

If you were going to express cause, this is not the word you
would use. Far better words are available, as you can verify
by the same means.

A reasonably direct translation is:
Every body perseveres in its state of being at rest or of moving
uniformly straight ahead, except insofar as it is compelled to change
its state by forces impressed. [Cohen & Whitman 1999 translation]

as quoted in http://en.wikipedia.org/wiki/Inertia

We see that /cogitur/ can be translated as "compelled". It could equally
well be translated as "forced". So Newton is saying that the body is
forced by a force to change its motion. To read this as a statement of
causation is a treeemendous stretch. It is even more of a stretch if
the statement is read in context, where on many occasions notions of
causality could have been asserted but are not ... or are expressly
disclaimed. On the very next line of the book, Newton uses a different
verb, /impelluntur/, which also means to drive or force, and (again!)
is not a verb commonly used to express causation. Also, presumably the
words /Hypotheses non fingo/ are known and understood by the folks on
this list.

One could also argue that the first law is unnecessary anyway, since
it can be considered a corollary of the second law.

I continue to object to the form of the argument. Moving bodies move
according to the *real* laws of physics, whether or not Newton or
Galileo or any other ancient "authority" gave a letter-perfect statement
of the laws.

To sum up: Appealing to Newton for evidence of causation is doubly
wrong. First, he didn't say that, and even if he did, it wouldn't
establish the facts of the matter.


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

Cliff Parker wrote:

ALL MODELS ARE WRONG. That does not mean that they are not useful.

Now if John or others want to argue that thinking that F causes A is wrong I would have to say he has a point. It is wrong, just like all the other ideas (models) we have in physics,

No! Not "just like".

It is a truism that we deal in models.
It is a truism that most (perhaps all_ of these models are imperfect.

The problem with truisms is that even though they are true, they are
not very informative. And to phrase the argument in Manichaen terms
(right versus wrong, black versus white) is to throw judgement to the
winds.

The crucial point is that some models are much, much better than others.
It is not our job to choose models versus no models; our job is to choose
better models over worse models.

So the issue is whether
"F causes ma (and not vice versa)" [a]
is better/same/worse compared to
"F equals ma" [b]
As far as I can see, every limitation and every correction that must
be applied to [b] must also be applied to [a]. I see absolutely no
logical or physical reason why anyone should prefer [a] over [b].

In contrast, there is every reason to believe that the physical
relationship between F and ma is symmetric, and is therefore better
modelled by a symmetric statement than an unsymmetric statement.
Equality is symmetric; causality is not.