Re: little gee and its sign

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 23:12 -0600 9/10/01, Jim Green wrote:

> I just can't believe that a physics instructor would tell his/her class
> that g is acceleration!!!!!
>
> I also don't understand why an instructor would introduce the concept of
> acceleration with free fall -- pedagogically this is nuts.

I have to agree with Jim on this issue. I have been campaigning for
years to eliminate the phrase "acceleration due to gravity" from use
in connection with "g." It is too easy for students taught with this
concept to come to the conclusion that *all* accelerations are 9.8
m/s^2! It is clearly *not* an acceleration, and the fact that some
objects fall with accelerations near that value is not relevant. I
think the most accurate description of g is that it is the
proportionality constant between mass and the force of gravity on
that mass.

The question is, what do we call "g" (not that it needs a partiuclar
name, but the students want one). I have opted for "gravitational
field strength," on the basis that it is closer to the truth than the
traditional name. Of course the actual value of g is not just the
strength of the gravitational field--it includes other effects, most
notably the effect of the earth's rotation. But those effects are
small, and if we have the students use 10 N/kg for g, the corrections
due to rotation, et.al., are much smaller than the round-off error.
(BTW, I prefer to use 10 N/kg for g, rather than 9.8, for
computational ease--it's much easier to do problems in your head with
the former number than with the latter--and the error introduced is
only about 2%).

This should not be too great a conceptual leap for students, and in
fact it should even make the dynamics of charged particles under the
influence of the electric force easier, since we have no problem
calling "E" the "electric field strength." In fact, using g as a
"field strength" gives one the opportunity to introduce the students
to a very deep concept--that of the difference between "gravitational
mass" and "Intertial mass." I even like to point out that charge can
be thought of as "electrical mass." The fact that gravitational mass
and intertial mass seem to be equivalent is not a priori to be
expected, just as the fact that "electrical mass" is not the same as
intertial mass (and hence does not cancel from the dynamical
equations when the electrical force is a factor). It isn't necessary
to make a big deal of this, but it is nice to point out to the
students that physics isn't always so cut and dried as they like to
think. And they also need to know that, in Lew Epstein's words, "You
don't have to get very far off the beaten path before you're in
really deep water." (HIs mixed metaphor, not mine.)

Hugh


--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they
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