Re: Centrifugal force

["Bob Sciamanda" <trebor@velocity.net>]

I think we are all (fortunately) spoiled by modern relativiistic models
and cannot easily put ourselves into the "purely Newtonian" way of
thinking.  To understand Newtonian mechanics I must rid myself of these
"misconceptions".

I think Newton believed in absolute time and space and, at the outset,
fully expected that the best that he might do would be to find a physics
which would work only for an observer at absolute rest.  The existence of
a whole class of inertial frames was an unexpected (and, I think,
undesired) conclusion, not an assumption.

He certainly believed in INVARIANT force pairs of interaction between
bodies as the basis of his mechanics.  Only the inertial observer can use
observed acceleration as the measure of these forces, since accelerations
are frame dependent, but Newtonian forces are frame invariant, existing
independent of observation or measurement.  When I speak of Newtonian
mechanics I try to recover his "misconceptions".

 We speak glibly about transforming forces between frames;  this was a
revolutionary innovation of Einstein and would not have occurred to a
Newtonian.  What was weird thinking before Einstein is now commonplace
and almost "self- evident", and vice versa - and rightly so - that is
progress.  "Misconceptions" is a relative notion - it depends on the
model which you are trying to understand.  Newtonian mechanics is still
very USEFUL (the only criterion we have), with all its "anachronisms".
We stand on giants on giants . . .   I prefer to give them due credit by
understanding them in the context of their milieu.  That is not just
history - it is physics, because it works!  (More comments below):

-Bob

Bob Sciamanda   trebor@velocity.net
Dept of Physics
Edinboro Univ of PA  http://www.velocity.net/~trebor
Edinboro, PA   (814)838-7185
-----Original Message-----
From: LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>
To: phys-L@atlantis.uwf.edu <phys-L@atlantis.uwf.edu>
Date: Thursday, April 30, 1998 10:02 PM
Subject: Re: Centrifugal force


> On 30/04/98 -0700, Leigh wrote:
>
> > .... In Newtonian mechanics an inertial frame is operationally defined
> > as one in which the law of inertia holds; it is as simple as that. ...

That is just one piece of the pie!  Not only must the law of inertia hold
for force-free objects;  accelerations must be accounted for by
(action/reaction) force pairs of INTERACTION BETWEEN THINGS.  This is
Newton's Force model and the heart of his mechanics.

> The law of inertia states that "if the net force acting on a body is
zero
> then the velocity remains constant". I do not see why this should not be
> applicable in the rotating frame of reference. If the bead on the
rotaing
> straw does not accelerate then the net force acting on it is zero.

The bead won't stay put on the rotating straw unless you provide an
unbalanced force (glue).
Why?

It was said that it is the "rotating frame" which is the agent? . . . !
What force-pair between a rotating carousel and a stationary tree causes
the tree to rotate as viewed by a carousel rider?  (In the Newtonian
scheme.)


> A frame of reference is inertial if it does not accelerate with respect
> to a frame which is already known to be inertial (the frame of distant
> stars). What is wrong with this operational definition?
>                                                        Ludwik Kowalski


Nothing!  This is a frame in which F=ma seems to work; but this is not
self evident a priori.  It requires the creation and testing of a model
of inter-object forces correlated to the observed accelerations.  The
whole pie sinks or swims (and is not necessarily unique).