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Re: CAUSATION IN PHYSICS



At 04:59 PM 10/14/00 -0700, Leigh Palmer wrote:
Real forces do
cause accelerations. If you've got any mathematically pristine point
masses around to use to check then you may have a problem determining
the causal relation between force and acceleration - if you succeed
in coupling your force to the particle! For extended objects forces
manifestly cause accelerations.

Then at 11:38 AM 10/15/00 -0400, Ludwik Kowalski asked:

What do you think about this argument, John?

0) To avoid misunderstanding and/or nitpicking: Let specify that I
interpret Leigh's statement to mean
Forces cause accelerations, and
not vice versa.

See item (2) for more on this.


1) To answer Ludwik's question: I see nothing to support Leigh's
conclusion. The only sentence that supports the conclusion is the
concluding sentence itself:
For extended objects forces manifestly cause accelerations.

When marking papers, I mark such things PbBA -- Proof by Bold Assertion.

The point may be "manifest" to some people, but it's not "manifest" to
me. There is no law of physics that requires it to be true. There is no
experiment that proves it to be true.


==============
2) The clarification in item (0) is important, because if somebody (e.g
Robert A Cohen, 07:26 AM 10/15/00 -0400) takes the position that

2a) forces cause accelerations, and by the same token
accelerations cause forces also...

that's OK. It's a slight stretch of the definition of causation, but I
don't have a problem with that. I have a slight preference for saying

2b) force is proportional to (not the cause of) acceleration, and
acceleration is proportional to (not the cause of) force

but the difference between version (2a) and version (2b) is too subtle for
the average student.

The non-subtle non-optional point is to recognize that F=ma is an
expression of equivalence (which is symmetric) -- the "=" operator does not
express any asymmetry between the role of F and the role of ma. None of
Newton's laws express any such asymmetry.

Some people may choose to control F (independent variable) and observe ma
(dependent variable) but that choice is arbitrary, and must be recognized
as arbitrary and recognized as not part of Newton's laws.