Re: little gee and its sign

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: "Larry Smith" <Larry.Smith@SNOW.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, September 12, 2001 11:01 AM
Subject: Fwd: Re: little gee and its sign


> At 11:12 PM -0600 9/10/01, Jim Green wrote:


> > I also don't understand why an instructor would introduce the concept of
> > acceleration with free fall -- pedagogically this is nuts.

Free fall IS the most available example of objects accelerating at a
constant rate and is therefore prime fodder for kinematics problems.  Almost
all of the other accelerations that are made up for kinematics problems are
unrealistic.  OK--we have to ignore air resistance with free fall, but when
I drop a bowling ball from 2-3 meters above the floor, air resistance _will_
be ignorable.


> I don't see where Tim said he _introduced_ the concept of acceleration
with
> free fall.  I don't, and I doubt he does either.  But it _is_ an important
> special case that merits discussion, IMHO.
>
> Larry

 From a simple Newtonian view:

a = net F/m

In free fall,    net F = GmM/r^2  (ignoring air resistance and rotational
effects)

a = GM/r^2   which IS an acceleration and IS usually called 'g'.

Admittedly the above is also the Gravitational Field which is also labeled
'g'---I usually use a script 'G' when dealing with the field, but we are
stuck with the Weight = mg notation, so we need to identify 'g' as the
field.  In other words, an unsupported object immersed in a gravitational
field 'g' experiences an acceleration of 'g'.  The dual nomenclature is too
prevalent to pretend we can shield students from it (although one can try
never to label the acceleration in kinematics as 'g').  Instead, I think it
is best to simply address the fact that 'g' is used in both contexts
(acceleration and field) and one must do a little mental work to separate
the two.  Nobody (at least no student I know) ever said that Physics is
easy!  ;-)

Rick

(Or we could just start off with the General Relativistic view of gravity!)

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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