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Re: Causation +- muscles



At 01:42 AM 10/22/00 +0100, J. Peter Vajk wrote:
Someone suggested that we not mention "causation" since Newton's Third
Law tells us that the forces between two bodies are simultaneous, and
that causation implies temporal sequence.

1) Yes, my recommendation is to say neither "F causes ma" nor "ma causes
F". The relationship between them is most accurately called "equality" not
"causality".
2) Temporal sequencing (or the lack thereof) is one reason, but not the
only reason for the recommendation.
3) I remain willing to accept the stretched version "F causes ma" _if_ we
also accept "ma causes F".
4) I also recognize that one can supplement the laws of motion with the
assertion "F causes ma and not vice versa" ... but I insist that this
supplementary assertion is arbitrary, unnecessary, and unscientific.

if I am standing at the chalkboard and decide to cross the room to look
at a student's work, my body must change its state of motion from rest
to moving at more-or-less constant speed across the room.

True.

The implied acceleration of my body must be due to an external force
acting on my body,

Saying "must be due to" at this point in the "proof" is grossly
fallacious; it assumes in advance the point one is trying to prove.
http://www.people.virginia.edu/~jjj6b/list.htm "begging the question"
http://www.fas.nus.edu.sg/philo/lecture_5.htm
http://www.psnw.com/~bashford/logic01.html "Petitio principii"
and/or "Circulus in demonstrando"

there is no doubt that my volitional thrust against
the floor is causative ultimately of my acceleration.

That statement is true! But note that it doesn't say anything about
force. It doesn't even mention the word. Therefore it cannot prove
anything about the relationship between force and acceleration.

In fact stronger statements can be made: in some situations human volition
seems to be connected more directly to muscular force than to muscular
acceleration. Do the experiment: take two smallish opaque jugs. Fill one
with sand so it is heavy, but not so heavy that it can't be readily
lifted. Leave the other jug empty so it is very light. Set them on the
table before an unsuspecting student. Ask the student to wait for the
count of three, then to quickly reach out and lift both jugs to shoulder
height.

You will observe that the light jug flies up much faster and
overshoots; the heavy jug comes up slower and undershoots. This indicates
that human volition is connected to muscle force more directly than it is
connected to position, velocity, or acceleration. Of course humans _can_
control position, velocity, and/or acceleration, but it is more indirect,
involving feedback loops. This is all quite understandable in terms of the
biophysics of nerve impulses and muscle fibers.


But this biology lesson proves nothing about the general, physical
relationship between force and acceleration!
a) It is one thing to demonstrate a connection between volition and
muscular force in a particular situation.
b) It is another thing entirely to claim that "force causes acceleration
and not vice versa" as a universal truth, and to claim that this is
required by the laws of motion.

Experiments provide evidence for item (a). There is not a shred of
evidence for item (b).