Re: is free-fall an inertial frame?

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- Original Message -----
From: Stefan Jeglinski <jeglin@4PI.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, August 18, 1999 1:16 AM
Subject: Re: is free-fall an inertial frame?


> > 1) Newtonian response:
>
> <snip>
>
>
> > In this paradigm, an object in free fall at the earth's surface is
> > impelled by the gravitational force of the earth.  In searching for an
> > inertial frame, this force must be taken into account.
>
> As you mentioned, in searching for an inertial frame, we look for a
> way to write F=ma. If we choose rotating frames, we quickly run into
> problems doing this. But it is easy enough to find a frame in which
> we can write F=mg.
> . . .
And you thereby recognize the real force, F=mg, of the Earth's
gravitational attraction on a mass m at or near the earth's surface.
This force does not go away when you remove all other (eg. supporting)
forces and send the mass m into free fall; m is then accelerating under
the influence of the still present force mg;  m is NOT in a force free
situation.  (All this is said within the Newtonian model/paradigm.)

The first law cannot be used in isolation to find inertial frames.  The
left hand side of the second law must be filled in before one can identify
a force free situation.  Isaac required the universal gravitational force
to account for the solar system, the falling apple, etc.  Thus, in this
model free fall is accelerated motion caused by an unimpeded gravitational
force.

Perhaps we are so imbued with GR notions that we now have difficulty
returning to the Newtonian way of thinking.  It would never occur to a
Newtonian, who has never heard of GR, to consider a state of free fall
acceleration to be force-free motion, constituting a Newtonian inertial
frame.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor