Re: Nitpicking: gravity is not a force???

[Hugh Logan <hlogan@ix.netcom.com>]

Hugh Haskell wrote:
> Ai Phing Wrote:
> > My students tend to think of g as the force.  And I have to keep
> > correcting them that it's acceleration, not force.  I use the words
> > "gravitationl pull" or "weight" for force though.  I never thought of
> > gravity as a field.  That really clears things up.  (Understanding comes
> > really late in life....)
> [snip]
>
> Be very careful how you use the language when talking about "g." Students
> hve more trouble with this concept than almost anything else in mechanics.
> Clearly "g" is not the force of gravity. That is "mg." But neither is "g"
> an acceleration, and this is one of the problems that students have. "g" is
> properly the strength of the local gravitational field (corrected for the
> rotation of the earth), and it should not be confused with an acceleration.
> The fact that a freely falling object (in a vacuum) near the earth's
> surface falls with an acceleration numerically equal to "g" is due to the
> fact that what we call "inertial mass" and "gravitational mass" appear to
> be the same thing (this is the first step towards Eintein's general theory
> of relativity), and so we can cancel the "m"-term out of the second law
> equation mg=ma. I know that I am fighting a losing battle in trying to get
> textbook to stop using the phrase "acceleration due to gravity" as the
> definition of "g" but I think this is why so many students are confused on
> this point (as I was at that stage as well). 

One high school text that makes the point that "g" (written as a vector)
is the gravitational field or gravitational force per unit mass is _PSSC
Physics_ (p. 244 in the 4th edition). The topics are sequenced so that
"g" is presented only after Newton's second law and the concepts of
inertial and gravitational mass have been introduced. 

Many texts introduce "g" as the "acceleration due to gravity" in
connection with free fall and projectile motion. These are presented in
chapters on kinematics -- before Newton's laws of motion and the concept
of mass have been presented. In this case, what else could one call "g"
?

Hugh Logan