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Re: what to call little "g"




I seem to have ignited a firestorm. I do enjoy this.

Herb Schulz wrote:

Sorry but "magnitude of the acceleration due to gravity on the surface of
the earth" is the correct title. The implication is that when gravity is
the ONLY force acting on the object the object will have an acceleration
with magnitude 'g'. If the name is too long for you that's why we use 'g'!



I have to respectfully disagree here. As Jim Green said, in dynamics
the "g" term always ends up on the "F" side of F=ma. As such, g
isn't an acceleration, but relates the gravitational force to the
gravitational "charge" (the mass). No acceleration is involved. For
the case of freefall one must not miss the step of mg = m a(sub)g.
Since the gravitational mass is equivalent in magnitude to the
inertial mass, then and only then is g = a(sub)g. This is a teachable
concept. I've been doing it for years, but with language that never
comes easily.

A story: a few years back an engineer I sometimes team-teach with was
holding a bob on a string, motionless. He pointed at the bob, asked a
student what the acceleration was. My student, looking at me, the
physicist, wanted to say (correctly) zero. But, he also knew that
this engineer wanted to "add" accelerations from gravity and the
tension. He was trapped and couldn't say anything.

The whole reason I raised this issue is that I'm searching for the
right phrase. I'm in a position, for the moment, to impact what may
be published, and I want to try to rectify the current fuzzy and
generally incorrect phrases used to name "g". So, my original
question is again, what should those phrases, designed to be used in
a textbook or other materials, be?

It has to be short. It has to be manifestly obvious what we're
talking about, and freshmen and younger must understand it. For that
reason I'm very uncomfortable with the word "field," for that's a
very subtle concept. As I said in my first post, there seem to be two
common uses for the symbol "g" that are mutually exclusive in
phraseology. One is the field strength (you physicists know just
what I mean, but our students will not) and the other is the
acceleration of an object in freefall.

From the discussion there's pretty much a consensus that the terms
as used now are misleading. What to do? Freefall acceleration for
the latter case seems easy to say. But what to call 9.8 N/kg? Are we
comfortable with local gravitational constant, even though it isn't
constant? (I guess the particle types out there won't be troubled by
variable constants...)


JEG
==================
John E. Gastineau
304 296 1966
Morgantown, WV
http://www.badgerden.com/~gastineau
email: gastineau@badgerden.com <==== NEW as of 10/1/96