Re: "acceleration due to gravity"

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

Kindly help me understand the difference between
        F=ma and F=mg.
 In the first equation  "a" stands for any
acceleration regardless of the force or mass
that may be involved?   In the second equation
"g" stands for the acceleration due to a
gravitational force resulting from a position
of a mass in a gravitational field?

On Thu, 25 Jan 2001 16:53:34 -0700 Jim Green <JMGreen@SISNA.COM> writes:
> 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

Herb Gottlieb from New York City
(Where it's nice to live but I wouldn't
want to be a tourist here)