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non-inertial and inertial frames




What is a force? As I understand Newton, and I haven't read the original,
is that which causes an object to deviate from motion in a straight line
at a constant speed (or a change in momentum) in an inertial frame,
presupposing the existence of, and knowledge of, such frames of
reference. Forces are measured by m*a in any inertial frame,

Actually, this is not so much a definition of force as it is a
definition of inertial mass. You do not need any kinematical
measurements to devise a force meter (the aforementioned spring
scales work perfectly well). Based on these measurements and
independent measurements of acceleration, a proportional relationship
is observed to hold. This is in fact the route by which Arons
advocates introducing the second law and the notion of inertial mass.

If this understanding is correct, force is tied to inertial frames. In
non-inertial frames, we can introduce apparent forces to account for the
acceleration inherent in the frame. General Relativity then transforms
gravity into one of those apparent forces.


Al Clark


This is one of the more popular misunderstandings of general
relativity, that it implies gravity is a fictional force loosely akin
to the centrifugal force (he said with trepidation). It is an easy
idea to acquire since the GR notion of gravity is so strange but it
is not quite correct. The gravitational force is perfectly real BUT
it is nonlocal. At a point, it is not measurable. Compare one point
to a point some distance away and then you can see the effects of
gravity.

This also gives me a chance to clear up a slight oversimplification
that has cropped up in this thread and is complicating the discussion
-- the notion that a free fall frame is an inertial frame in
relativity. It is not. It is LOCALLY an inertial frame but it is the
mismatch between free fall frames for separated observers that
constitutes spacetime curvature and so gravity.

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
(803)349-2227 The Name of the Rose
fax: (803)349-2926