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Re: Apparent weight




I'd understood what Dave Bowman was getting at, and was happy to see it
confirmed. I restate it just to check that I've got it right: When we take
the local surface of the earth to define our reference frame, instead of a
free-fall frame, the mg force appears, since we are accelerating in
space-time. The mg force is thus like the force that pushes me back in my
seat in the 747 accelerating down the runway. And both of these forces can
do work, even though we may choose to call them fictitious or inertial.


Limiting the question for the moment to the case of the experiences in the
747, may I ask why there is a necessity of reporting any other force than
the one that is felt on your back, exerted by the seat? Imagining
another force that you don't feel seems to me weird, and the fact that the
new "force" has no third law counterpart would seem to complicate teaching
students an understanding of Newton's laws. How do you handle that
aspect?

I also appreciate A.R Marlow's expositions, and would like to understand
better why he objects so strongly to this interpretation. *Is* it purely a
matter of definition? Presumably he does not want us to refer to forces as
causing accelerations unless measured in an inertial reference frame. This
makes him a pure centripetalist. Is this a personal preference only,

I certainly hope not; actually, if I had my druthers I would prefer it
otherwise. But reality deals hard blows sometimes, and we don't get our
druthers. The fact simply is that accelerations that are detected *only*
because one is measuring relative to a noninertial frame need no other
explanation than our choice of a reference frame; creating another
explanation (such as fictitious force) destroys the beautiful fabric of
both Newtonian and Einsteinian physics.

or is
there something in GR that makes this approach better? What becomes of the
concept of a "force" in GR?


Now you have hit a good fundamental point. I suspect that at the root of
much of the disagreement about fictitious forces and frames of reference
is a misconception about GR that should be added to the list of
misconceptions that was circulating earlier. The misconception is that
Einsteinian physics makes all reference frames equal. I have seen this
statement in at least one brief description of GR in a freshman physics
text, but it is simply not true. It is true that Einstein at one stage of
his long progress toward GR thought he might accomplish this, but (sadly?)
the theory he finally arrived at does not allow it. In both Newtonian
physics and Einsteinian physics there exists a privileged class of frames
-- the inertial frames -- and only particles not accelerating relative
to such frames can be said to be force free. The acid test, then, for a
*real* force is "Does it cause deviation from local inertial motion?" If
it does it is a force; if it doesn't it is not a force. And this holds in
both Newtonian and Einsteinian physics.

The only peculiarity of Newtonian physics is that Newton did not conceive
of curved spacetime, and so he misidentified inertial frames slightly: in
the vicinity of the earth's surface the error is only ~ 9.8 m/s^2, so
that's pretty good for what he had to work with.


A. R. Marlow E-MAIL: marlow@loyno.edu
Department of Physics, Box 124 PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453