Re: non-inertial frames

[Malcolm Davis <davis@nod.bms.com>]

> > > > > "AR" == A R Marlow <marlow@beta.loyno.edu> writes:

    AR> Sorry, but the term "fictitious" is not at all a misnomer when
    AR> applied to the psychologically created outward directed
    AR> centrifugal force in the examples you cited.  We do not feel
    AR> these forces.  What we feel is the inward directed (i. e.,
    AR> centripetal) pressure on our skin, and this is all that
    AR> exists.

> > > > > "al" == al clark <al@sci.nccu.edu> writes:

    al> You do not feel a force pulling you out, ie. a centrifugal
    al> force, What you feel is the force pushing you in. ...

Let me rewrite Al's paragraph thus

	You do not feel a force pulling you down, ie. a gravitational
	force, What you feel is the force pushing you up.

This statement is equally correct.  However, does this mean that
gravity is a "fictitious" force?  Well, maybe from a GR standpoint it
is.  Nonetheless it is quite useful to work in a Newtonian reference
frame where the gravitational force on an object is quite "real",
i.e. must be accounted for in F=ma.

The same is true for the other "fictitious" forces.  For example, it
is much simpler to handle the problem of the ballistics of a long
range artillery piece using a non-inertial frame at rest with the gun
and include the Corriolis force, than to work in the frame where the
rotation of the earth must be accounted for explicitly.

I must admit that I shudder when physics teachers get rolling on the
fictitious-ness of these forces.  It is true they are constructs of
our mind trying to make sense of a non-inertial frame, but that does
not make them "wrong" or "useless" in spite of those being some of the
general connotations associated with "fictitious".  So while students
need to come to see how the forces simplify (every force has a source)
when we view the situation from an inertial frame, exorbitant efforts
to "prove" the fictitious-ness of these forces I believe merely
"prove" that physics is not related to the students' reality.

So, I'm not sure whether there is a better word that fictitious, but I
think we need to make sure that, whatever we call them, we not portray
these forces as "wrong" or "useless", for neither is true.


Malcolm

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