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Re: [Phys-l] Equations (causal relationship)



I didn't suggest that one cannot add acceleration vectors. In the context of
this thread, the suggestion was made that F1, F2, and F3 acting on a
(assumed) point object are associated with an acceleration a that can be
decomposed into a1, a2, and a3. I totally agree one can do that. However, I
don't see a physical reason for doing it because an object has only a single
geodesic. The decomposed accelerations can certainly be produced, but seem
meaningless. Why not add additional acceleration pairs to the decomposition
related to force pairs that add to zero that weren't even applied to the
object in the first place. Again, just because you can do it doesn't mean
something useful is produced.

BTW, I come down on the side of this discussion that does not assert a
causal relation between force and acceleration. I see N2 as descriptive -
not causal. When a ball hits a wall, I find it difficult to assert that a
force had to come about because the object was decelerated. I don't see how
you separate the force and the acceleration - they appear to be too chicken
and egg.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Monday, May 01, 2006 10:23 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Equations (causal relationship)

Bob LaMontagne wrote:
I have a conceptual problem with this. I can conceive of a situation where
there are many identifiable forces (tension, gravity, wind resistance)
acting on an object, but a point mass has only one acceleration - the
second
derivative of its position - and it has only one position. There are
certainly accelerations it might have had if each of the forces acted
individually - but they don't.

I think it's simply bad pedagogy to set up a net acceleration that is
defined as a sum of a collection of phantom accelerations

There's nothing phantom about it. The car accelerates relative to
the table, and the table accelerates relative to the floor. The
acceleration vectors add.

I think it is simply bad pedagogy to suggest that the vector-addition
law applies to some vectors and not others. I think it is simply bad
pedagogy to suggest that the substitution property of equality applies
to some quantities and not others.

Mathematical axioms and bedrock principles are not to be trifled with.

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