Re: A weighty subject

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

> On Fri, 15 Oct 1999, Joel Rauber wrote:
>
> > I'm convinced to ignore things like buoyancy in the
> definition.  But in
> the
> > elevator accelerating upwards, in the frame of reference of
> the elevator,
> > there is an inertial force upwards as well as a normal
> force upwards on
> the
> > object being weighed on the scale.
>
> No. From the Newtonian viewpoint, there is an inertial force downwards
> that adds to the gravitational force making the normal force larger.

Oops! Absolutely correct, I reversed a crucial minus sign in my statement.

> > Do you really want to call that inertial force a part of the
> > gravitational force.
>
> With the correction above, yes, I do.  But for my
> introductory students
> no.  I would stop at saying that the object's weight is equal to the
> magnitude of the normal force, e.g., m(g+a).

John, I want to pursue your viewpoint a little further. (Leigh chime in here
as you will, because I read your reply to me as meaning that you *do not*
want to call the weight in the above example "m(g+a)", as John seems to do,
and me I might add).

We agree on what inertial forces are (at least when I finally get the
directions correct.)

You want to call *all* inertial forces gravitational. Correct?  Why?

I want to say we can categorize inertial forces into types, and reserve the
name gravitational force for the inertial force that one would commonly
calculate to be mg near the surface of a planet.

I suppose you might object to my categorization by saying, how would I tell
the difference (locally) between these different types of inertial forces,
the "gravitational" one from the "Coriolis" one from the "elevator" one. I'm
not sure, I'll have to think about it.  But there is a difference globally,
namely the one I wish to put in the "gravitational" category, involves
properties of other objects, namely their gravitational mass; the property
they have which bends space-time.

On the other hand if you present me with a non-minkowskian space without
other objects, I'd say the global property is the intrinsic curvature of the
space, something that the inertial force I categorize as "coriolis", or the
one called "elevator" doesn't have. So I think we can distinguish them and
put them into seperate categories which are proper subsets of the category
inertial forces.

Joel Rauber

eagerly awaiting the clarifications that will help more understand the
issues involved in a more enlightened fashion.  I.e. help me correct my
mistakes and misunderstandings.

Gotta go to class, which will soon bring up a new topic that I want to ask
the gang about.