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The problem with "frame-dependent" accelerations is that they are just 
artifacts of the frame you chose to use to define the accelerations and 
therefor the forces that must have caused the accelerations are also 
arbitrary artifacts too. Since anybody can choose any gyrating, 
spinning, hopping, twirling etc., accelerating frame of reference, any 
measured motion of an object could have an infinite number of different 
forces and accelerations. If I get to choose the right accelerating 
frame I can prove that it was your nose that hit my fist. But for non 
accelerating frames of reference, inertial frames, it doesn't matter 
which frame you choose the observer will always calculate the same 
forces for any observed object. I understand that there are some 
computational advantages in some circumstances to create fictitious 
forces when dealing with rotating frames such as the earth.  For 
example, if I am demonstrating a loop to a flight student it is easy for 
me to tell him that it is centrifugal force that presses his butt into 
the seat at the top of the loop while we are upside down. This is easier 
than the real explanation that pulling back on the stick while we are 
upside increases the angle of attack of the wings causing the wings to 
create a greater force. Since we are upside down this greater force from 
the wings (lift) causes the aircraft to accelerate towards the earth at 
a rate of acceleration greater than than the rate of acceleration caused 
by gravity thereby leaving the pilot's behind behind. Even though this 
is convenient, it that still doesn't make centrifugal force real since 
it is just another fictitious force invented to allow a simple 
explanation in the looping frame of the airplane.

gl


..


frankreed@HistoricalAtlas.com wrote:
> Quoting Gary:
> "Coriolis is a fictitious force use to explain the perceived curve of the 
> flight path as observed by an observer on earth of other rotating frame 
> of reference."
>
> Coriolis acceleration is a "frame-dependent" acceleration. That is, it 
depends on the choice of coordinates that you use to define motion. The 
expression "fictitious force" is a technical term intended to describe this 
frame-dependence. Just so there's no misunderstanding, it DOES NOT mean that 
the Coriolis acceleration is "fictional" in the ordinary sense of the English 
word. Of course, Coriolis acceleration is "real" in the sense that in many 
circumstances you find that the natural coordinates to describe the problem 
are rotating coordinates. The weather on Earth or any other planet, for that 
matter, is nearly impossible to understand in non-rotating coordinates but 
quite easy to understand in coordinates which rotate.
>
> >From a modern perspective (modern = since the early part of the 20th 
century, after the development of general relativity), even the common 
acceleration of gravity near the surface of the Earth (at a single point) at 
9.8 m/s^2 or 32 ft/sec^2 is a "fictitious force" since it can be eliminated 
by going to a frame of reference which is accelerating toward the Earth's 
center, in other words a freefall frame. And sure enough, if you place an 
aircraft on a parabolic trajectory with that acceleration, gravity disappears 
and passengers are rendered completely weightless exactly as if they are in 
orbit (until the plane's acceleration trajectory is changed). It is not "as 
if" there is no gravity; in that frame of reference, there really is "no 
gravity" apart from local tidal accelerations.
>
> Many people are able to explain the origin of the Coriolis acceleration by 
describing how it appears in an inertial, non-rotating frame of reference. In 
such a frame, the object moves on a straight line "while the Earth turns 
beneath it". This is very important information, of course, but it is a 
derivation, like an "etymology". It tells us why this acceleration must exist 
in non-inertial frames of reference, but it doesn't mean that it is fake or 
"fictional". And you surely wouldn't want to revert to the derivation once 
you understand why it works. 
>
> In many ways, the expression "fictitious force" in physics has caused as 
many problems as the expression "imaginary number" in mathematics. Neither of 
these concepts are invalid or fictional or "un-real". For modern mathematics, 
so-called imaginary numbers have no defects, no lack of reality to them. They 
are what they are: solutions to algebraic equations. Likewise in physics, 
so-called fictitious forces are not forces to be avoided or treated as 
"un-real". They are what they are: forces arising from the choice of 
coordinates.
>
> -FER
>
>
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> >
>
>   


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