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Re: Weight and Mass



force of gravity of the earth, as it is normally used, that is, the
reading of a scale when both scale and user are at rest and firmly
planted on the earth, includes the effects of the earth's rotation on
that reading, which varies with latitude.

What we need to do is change the concept of *normally used*. Terminology
is >sufficiently bastardized so as to introduce a great many unnecessary
complications. >Better to pick one, the correct one, and stick with it no
matter what. Many textbooks >are finally getting this clue.

We have, of course, been through all this before. And I firmly agree that
it is good to pick one definition for each term. The challenge is that
physicists tend to be an independent breed. That means various people may
develop similar ideas independently. This can lead to 1) the same name for
different ideas, or 2) different names for the same idea. The challenge is
then to agree on one consistent set of terminology - to decide which
definition constitutes "the correct one".

Consider the following useful ideas (avoid/ignore buoyancy, holding on to
ropes, etc, etc):
a) the gravitational force between you and the earth
b) the reading when you stand on a scale on the earth
c) the reading when you stand on a scale in any other reference frame.

All of these clearly have something to do with weight, so which one should
we call "the weight"? ANY of the following sets can be logical and
consistent names (respectively):

SET I
a) force due to gravity
b) weight
c) weight


SET II
a) force due to gravity
b) weight
c) apparent weight

SET III
a) weight
b) apparent weight
c) apparent weight



My top choice (and I *think* the consensus of the group here) has been the
first. My reasons are as follows.
1) F(G) = "force due to gravity" is a perfectly good and clear name for
the first concept. Why use a second name to mean exactly the same thing?

2) If W == mg == m(9.81 m/s/s), then this numerically is not GmM/r^2, but
rather GmM/r^2 - mv^2/r. The standard "g" has the earth's rotation built
into the definition. What we most commonly calculate as weight on earth is
the same as (b).

3) Since weight (in in the previous point) already is the "apparant force
of gravity" in the specific frame of the rotating earth, the word
"apparant" seems implied. So why add it specifically in some reference
frames (e.g. in orbit), but not in others (e.g. on the rotating earth.)?

Does anyone want to argue for a different set of definitions for the above
concepts???


Tim Folkerts
FHSU