Re: Apparent weight

[Mark Sylvester <msylvest@spin.it>]

> ------------------------------
>
> Date: Thu, 19 Feb 1998 16:04:48 -0600 (CST)
> From: "A. R. Marlow" <marlow@loyno.edu>
> To: phys-l@atlantis.uwf.edu
> Cc: dbowman@tiger.gtc.georgetown.ky.us
> Subject: Re: Apparent weight
> Message-ID: <Pine.A41.3.96.980219151315.129870B-100000@nadal.loyno.edu>
>
> On Thu, 19 Feb 1998, David Bowman wrote:
>
> > ...My point was a
> > counterexample that hydroelectric utilites are in the business of selling
> > electric power (i.e. work) produced by the action of falling water which
> > is pushed downward by the earth's gravitational field (mostly usual
> > Newtonian gravity with a small negative contribution from the centrifugal
> > force field from the earth's rotation) that *exists* (locally) by virtue
>                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > of our choice of a frame in which the earth's surface is taken to be at
>  ^^^^^^^^^^^^^^^^^^^^^^^^
> > rest rather than using a freely falling frame.  
>
>
> Now we are at the nub of our difficulty:  I say only accelerations come
> and go by virtue of our choice of frame, while you say that forces also
> come and go in the same way.  To me, the choice of a frame for describing
> motion and the accelerations that can be measured relative to a chosen
> frame are matters of kinematics.  For you, apparently, it is also a matter
> of dynamics, and you are willing to say by definition that a force comes
> into existence every time I start measuring acceleration relative to some
> noninertial frame.  Definitions are free, so of course you can do that if
> you wish, but you will pardon me I hope if to me it seems madness.  

I'd understood what Dave Bowman was getting at, and was happy to see it
confirmed. I restate it just to check that I've got it right: When we take
the local surface of the earth to define our reference frame, instead of a
free-fall frame, the mg force appears, since we are accelerating in
space-time. The mg force is thus like the force that pushes me back in my
seat in the 747 accelerating down the runway. And both of these forces can
do work, even though we may choose to call them fictitious or inertial. 

I also appreciate A.R Marlow's expositions, and would like to understand
better why he objects so strongly to this interpretation. *Is* it purely a
matter of definition? Presumably he does not want us to refer to forces as
causing accelerations unless measured in an inertial reference frame. This
makes him a pure centripetalist. Is this a personal preference only, or is
there something in GR that makes this approach better? What becomes of the
concept of a "force" in GR?

We've been over this before (cows in the field watching the train accelerate
come to mind), but I feel like that student who always asks me the question
that I've just finished answering.

Thanks, Mark.

Mark Sylvester
United World College of the Adriatic
34013 Duino TS
Italy.
msylvest@spin.it
tel: +39 49 3739 255