Re: "acceleration due to gravity"

[Jim Green <JMGreen@SISNA.COM>]

Oh my, I had no idea that my comment would stir things so much,  I did not
dream that there would not be a clamor of agreement.

My position is quite simple: "g" should not be called "acceleration due to
gravity" because it is not acceleration!  "g" represents the local
gravitational field -- and its introduction should be accompanied by a
carful explanation of the concept of the artificial mathematical invention
of a field.

One does not need Leigh's complications to see the necessity of making the
use of "g" clear to the students.  If the student sits stationary in the
lab, "g" is not zero but there is no acceleration -- at least no local
acceleration.  "g" represents only one of many possible forces on a
system.  Now it might be made very clear to the student that the
acceleration of a falling object -- call it a-sub-g -- approximates
Newtonian gravity only in this very narrow case -- and is not a very good
approximation at that -- especially if other gravitational fields and the
Earth's rotation are taken into account.   it certainly is not the way "g"
is measured over the Earth' surface.

It seems to me that precise language in a physics class is pedagogic.  If
the instructor is careful in his/er language, students learn from that
precision --  concepts are reinforced.  The only motive I can see for such
sloppiness is laziness of thought -- indifference to the students and
subject matter.

"g" should be introduced via Newton's "Forth Law"  not during
Kinematics  -- In fact the common laboratory experiment of dropping an
object, measuring its position v time and then saying Ah ha I have
determined "acceleration due to gravity" -- ugh I am embarrassed to have
said it out loud -- this experiment is counter-productive.  It is commonly
done, I guess, because everyone else does it.  _I_ don't do it; I use an
air track and _later_ an Atwood's Machine -- where Newton's Laws can be
explained in a way that the students understand rather than are confused
and where the use of the term "acceleration due to gravity",  ie a-sub-g,
then might make sense, but I would still hold that the term is always
counter-productive.

[Leigh says as follows:]

> As Chuck Britton only slightly jocularly reminded us, the "acceleration of
> gravity" commonly subsumes the centrifugal acceleration. It is a term
> appropriate only to the laboratory frame on Earth's surface, a quantity
> which can be measured by students in the lab. I guess Jim's point is that,
> since it is not really a "purely" gravitational acceleration, we shouldn't
> call it that.

Leigh, it is not acceleration at all!!!

> However, in the context of the principle of equivalence, we
> recognize that there is no real distinction between the two components in
> the lab frame. There is no way to measure how much is "purely" gravitational
> and how much is "fictitious". g (a vector quantity) is the gravitational
> field in the laboratory. It manifests itself as the initial acceleration of
> any body falling freely from rest in the laboratory frame.

Just little me over here in my corner of the galaxy.

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen