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Coriolis, etc.

As an oceanographer, I feel some sort of duty to interject something
here.

A. R. Marlow <marlow@beta.loyno.edu> feels strongly that the word
"force" is improper to use when referring to phenomena, such as the
Coriolis effect (clever sidestepping, eh?), that are observed only in
non-inertial reference frames. It seems to me, though, that one
important consequence of general relativity is that there are *no*
preferred reference frames for doing physics: non-inertial frames are
just as valid as inertial frames, as long as we're explicit about
precisely what kind of reference frame we're dealing with.

To put it another way: Galilean relativity tells us that a man in a
windowless carriage moving at constant velocity (relative to an
inertial frame) is perfectly correct when he says that he is
motionless, and when he does his physics accordingly. General
relativity tells us that a man in a falling elevator is correct when
he says that there is no gravitational force (except the tiny forces
due to his mass and the elevator's mass) observable *in his reference
frame* and does his physics accordingly. Likewise, a man in a
windowless, accelerating rocket ship is perfectly correct, in a
general-relativistic sense, if he chooses to say that *in his
reference frame* he observes a "downward" force that acts on all
objects, and that the strength of that force is proportional to a
given object's mass. We (in an inertial frame) could re-write his
equations, substituting the rocket's acceleration for his observed
"force," but our equations are no better than his. We observe his
spaceship to be accelerating; he observes a force on all the objects
in it.

In that case, it should be perfectly acceptable to take the surface of
the spinning Earth as our reference frame, and to formulate our
equations accordingly. Hence I can say (with only a little
trepidation) that *in this reference frame* we observe a force on any
moving object, and that the force is proportional to the object's
velocity (relative to the Earth's surface) and to a parameter known as
the Coriolis parameter. Of course, it would be just as valid (although
much more difficult logistically) to work from an inertial reference
frame and then to transform our (Coriolis-free) equations to the
coordinates of the spinning Earth.

I disagree when Marlow says that the "fictitious" forces "will not do
any of the things expected of forces in ANY reference frame." In my
spinning reference frame (his too, by the way), Coriolis acts
*precisely* as a force. (Although one difference between, say,
Coriolis and gravity is that we know precisely how the Coriolis force
originates--from our bizarre choice of reference frame--and we don't
really understand *what* causes gravitational forces.)

This is why all oceanographers (and meteorologists) believe so
strongly in General Relativity. :*)

Ari Epstein

P.S. I agree that what we have here is primarily a linguistic
argument; it depends what we mean when we say the word "force."