Re: Apparent weight

["A. R. Marlow" <marlow@loyno.edu>]

On Sat, 21 Feb 1998, David Bowman wrote:

> A. R. continues:
> > Well then, according to your weaker criterion, forces become
> > accelerations;  why use the two distinct terms?  Why teach our students
> > two distinct terms if they really always mean the same thing?  How
> > would you distinguish the two terms pedagogically?  I am really asking
> > this to gain enlightenment on how to teach these matters in a practical
> > way.
>
> These are good and fair questions.  To answer the first part, I prefer 
> to use the concept of forces as the mathematically functional 'causers'
> of accelerations in any given frame.

So am I correct in concluding that in your approach *any* acceleration
relative to *any* frame always has a force corresponding to it?  To be
specific, since I clearly measure myself to have big accelerations
relative to yonder stunt pilot doing maneuvers in the sky, if I wish to
calculate in that frame I gain a force acting on me that I do not feel,
but which "mathematically functions" as the cause of the accelerations?
Why does mathematics require such a cause?

> Regarding the substantive last
> question, the way I see the situation is that the answer is related to a
> frequent/usual reason why the postulated existence of any forces are
> invoked in the first place.  Usually the way mechanics is presented to
> students is in the context of the presentation of Newton's laws.  First,
> we tend to emphasize to students via N1 that the natural condition for
> influence-isolated objects to be in is a state of coasting.  That is,
> things tend to keep doing what they have been doing unless compelled to
> do otherwise by some external agency.  We then call forces those external
> influence agencies that act to change the state of motion of objects.

I can't help but notice that you make no mention, here or in all that
follows, of inertial reference frames or inertial trajectories (the
geodesics of either Newtonian flat or Einsteinian curved spacetime).  Is
that by design.  Do you ever include such in your teaching.  If so, I
would be intrigued to hear how.  

> ...
> Since I don't take the ontology of forces that seriously in the first
> place, I tend to gravitate toward the functionalism of the N1 & N2
> priority over N3 senario, where forces are just the *mathematically
> functional* motivators of accelerations rather than actual necessarily
> ontologically existing influences between pairs of pieces of matter and
> between different spatially adjacent regions of interaction fields.  IOW,
> I don't necessarily see forces as actual causes of anything, just as
> mathematically useful constructs that mathematically function as such
> causes on individual objects.
>
> David Bowman
> dbowman@gtc.georgetown.ky.us

Very interesting.  Now I see where the big differences in approach arise
from.  To me forces are very real and powerful physical phenomena, whose
properties and effects can be *described* mathematically. I don't conceive
their existence as purely mathematical, and so I take there ontology very
seriously, as I am sure you have noticed from previous posts. I thank you
for your candor and clarity.  It at last clears up where the differences
lie.


A. R. Marlow                          E-MAIL:  marlow@loyno.edu
Department of Physics, Box 124        PHONE:   (504) 865 3647  (Office)  
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