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Re: Weight



meanwhile. As far as I know gravitational weight is the only way to teach
in UK , Australia, South Africa, Western Europe (?) I think, Russia is
practicing an operational definition. I am writing this message as I was
positively surprised to discover views, similar to mine, on this subject of
weight in some contributors to this important forum. Since 1992 I am
constantly advocating operational weight definition in my talks at AAPT
summer meetings. Some support in the beginning from Mario Iona was
important. We see that the story is not finished.
I agree and have been advocating this for years. Just as we don't give a
special name to electrical force, why do we need to give one to the gravi-
tational force? Also, since we define weight as mg in most textbooks, which
g are we talking about? The one defined as GM/R^2 or the one for the
rotating earth?

In 1980 I made a transition from solid state physics to astronomy - I came
out of the closet, so to speak. One of the things that had always deterred
me from astronomy before was the weird and wonderful welter of names and
arcane conventions. Astronomical objects were identified by a variety of
schemes devised by people dead for centuries. There were constellations,
Bayer designations and Flamsteed numbers, Messier and NGC (for "New General
Catalog" which is neither new nor general) numbers, and, quaintest of all,
a brightness quantification scheme based not on physical measurement, but
rather on a subjective method devised more than 22 centuries ago. In 1980
we made photometric measurements by photoelectric techniques, with standard
filters, and *still* we used a scale designed to be consistent with one
used first in classical Greece*!

Well, this was, to a physicist, clearly a silly situation. Now that we know
more than the classical Greeks did surely it is time that we reform all
archaic systems of nomenclature and measurement. We should sweep away these
conventional dinosaurs and bring astronomy, the most venerable of sciences
in which we all find our own roots, at least into the twentieth century! At
the very least astronomy must be made to use SI.

Well folks, I quickly learned that my attitude was born of ignorance. The
retention of venerable conventions has had an indispensible result: the
literature of astronomy is accessible to astronomers today without the use
of conversion tables and a dictionary of obsolete terms. Observations of
ephemeral phenomena made by astronomers long dead can inform astronomers of
today. They are an irreplaceable resource, and accessibility to them is
facilitated by retention of these "outdated" conventions.

What is being proposed here is that the meaning of the word "weight" be
changed in just the way that I pointed out that the meaning of the word
"mass" has been changed. Henceforward many of you would like to have the
word mean either the (scalar) reading on a spring scale or else the
(vector) force of reaction which acts on the object being weighed. Old
codgers like me, who have always taught and written that weight is the
gravitational force which acts on a body would then, retroactively, be
declared to be "wrong", or at the very least would no longer be useful to
be used as references by students schooled in the "modern" conventions.

I think that what I am seeing here is an inappropriate reaction by some
of my colleagues to a frustrating situation. We are all finding it to be
increasingly more difficult to present the concepts of physics to our
students successfully. "Weight" (as I have defined it) is a simple concept
which, somehow, it appears an increasing number of students are incapable
of assimilating. What you revisioists are suggesting it that we offer them
a concept which is (you hope) more readily assimilated, and you intend to
call it by a name which is already in use. Clearly the pressure has
affected your judgment if this seems reasonable!

I would like to suggest an alternative way to view the problem and its
solution. Physics used to be an elite subject. When I was in high school
(in the late Pleistocene) the brightest students all went into physics or
EE in college. The luster of the Manhattan Project was still high, and the
students who went into physics had no trouble with the concepts as they
were taught then. Perhaps the reason that we are having difficulty
teaching these concepts to today's students is that they are not the
brightest high school students any more! I've looked hard at this
hypothesis. I haven't been able to invalidate it. What do the rest of you
who have been teaching three decades or more think about that idea?

Changing the conventional meaning of "weight" is an inappropriate reaction
to a pedagogical "problem" that doesn't exist. If a student proves
incapable of understanding the conventional definition then perhaps we
should advise that student that he is not suited to a discipline which
requires him to understand physics. Again, I'll have to apologize for
offering such a radical solution.

Leigh

*I refer, of course, to the scale of stellar magnitudes first used by
Hipparchus in his star catalogs.