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Re: Apparent weight



Hi all-
In response to Mark Sylvester's
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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.

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, or is
there something in GR that makes this approach better? What becomes of the
concept of a "force" in GR?
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I seem to have a somewhat different view of Newton's Laws, so I'll
take this opportunity to record them. This view is somewhat closer to that
espoused in MECHANICAL UNIVERSE by Olenick, et al. The problem is to keep
the three laws from being trivial:
N1: There exist inertial frames. The test for an inertial frame is that
an object that is not acted upon by a force is not accelerated.
Comment: This leaves the notion of force undefined. It also insures
that N1 is independent of N2.

N2: F=dp/dt
Comment: The 4 forces of nature are given. N2 then permits one to
measure mass (definition of mass). Otherwise, N2 is empty because
force and mass are defined in terms of each other.

N3: Any part of a system may be analyzed in isolation, using N2, replacing the
remainder of the system by the forces exerted by that remainder.
Comment: N3 is just a computational device.

As far as GR is concerned, the key point is that one can always find
a local inertial frame in any curved universe. In that frame Newton's
laws, being local laws, take on the same form that they do in special
relativity. The problem of GR is how to get from one local frame to
another one. This view is especially emphasized in Misner, Thorne and
Wheeler's book.
Regards,
Jack