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



On May 1, 2006, at 7:23 AM, John Denker wrote:

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 John D. may be being a bit disingenuous here. Bob seems to be responding to JD's previous suggestion that one can add accelerations due to different forces acting on the same object just as one can add the forces themselves. But JD seems now to be changing the subject to one of relative accelerations which nobody will disagree are additive.