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Re: centrifugal force



Come now fellas. This argument seems to be getting a little harsh.

By "agent" he means another body as a source of the force. For
"centrifugal" force there is no such body, unless you want to believe
that the rotation of the stars around the Earth causes the effect.

Why are we making a mountain out of this? The definition: "An inertial
frame is one in which the law of inertia holds" is circular! How do you
know if the law of inertia holds? You have to decide whether a body is
force free. How do you do that? Well if it doesn't accelerate? But then
you are assuming that you are in an inertial frame!

It was clear many decades ago that there is no absolute way to decide
whether you are in an inertial frame. All you can do is the following:
If you find an apparent force (i.e. a deviation from rectilinear uniform
motion) then test to see if that "force" automatically adjusts so that
the acceleration is independent of the mass of the body. If it is, then
the force can be transformed away (locally at least) by assuming an
accelerated motion of your reference frame. Thus, forces that have that
property (acceleration independent of mass) can be regarded as
fictitious forces. Centrifugal and Coriolis forces share that property,
as DOES GRAVITATION. That realization is what led Einstein to the
Principle of Equivalence and the theory of General Relativity.
Gravitation can be transformed away (locally) by changing the reference
frame (to one in free fall). Then no experiment will distinguish your
situation from any (other) inertial frame -- until you hit the ground
(which from your point of view was accelerating toward you). It is this
universal property of gravitational acceleration that allows it to be
interpreted as a geometrical phenomenon and thus paves the way for
General Relativity.

It is from the theoretic point of view arbitrary to decide whether these
"inertial" forces are real or not, but is useful to do so in order to
maintain that there is something definite about the idea of inertial
frames. SOme of my research colleagues (relativists mostly) still do not
agree on whether one can speak in absolute terms of accelerated frames.
There is a subtle distinction between centrifugal and Coriolis effects
on the one hand and gravitation on the other, . . . . but that's enough
for now.
Jerry Epstein