Re: "acceleration due to gravity"

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

I have been campaigning to stop calling "g" the "acceleration due to
gravity" for years. Just as you have, I have found it
counterproductive. In fact, even though I don't use it that way,
their text does (I suspect that editors, all of whom have learned
physics in the same dumb way, don't know any better and keep telling
the authors that they *have* to do it the old way or nobody will buy
their book--heaven forbid, that we might ask the physics teachers to
think a bit. After all, we don't expect the students to think, so why
should we expect the teachers to do so), and so they still get the
idea no matter how hard I try to press it out of them, and every year
a certain fraction of my students complete the course thinking that
"all accelerations are 9.8 m/s^2! It really makes for some
interesting answers to problems.

I had just about given up ever seeing a significant fraction of
physics teachers adopting the "gravitational field" approach, but it
actually looks like we "right thinkers" are approaching a critical
mass, and might actually carry the day--some day.

Thanks for raising the issue again, Jim. It needs a re-airing every so often.

Hugh

> 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.
>
> Jim Green
--

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

(919) 467-7610

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