Re: acceleration of gravity

[Herb Schulz <herbs@interaccess.com>]

> I object to the notation of "weight" and "apparant weight."
>
> I like to define weight as the reading one gets when standing on a spring
> scale.  Your weight is then  mg only when the acceleration of gravity at your
> location is "g" and you are not accelerating.  Among other things, this leads
> to a nice development of the concept of "weightlessness."  It's not that the
> forces go to zero, etc.

Howdy,

I guess I'd call what a scale reads that the "apparent weight!" That's
because the scale only reads the force *it* is applying on the object *not*
the force due to gravity on the object.  The weight of an object is the
force due to gravity on the object: the apparent weight is the force the
direct surroundings apply on the object.

Put the scale in an elevator that is accelerating and it will no longer
read the object weight.

I still call 'g' "the magnitude of the acceleration due to gravity near the
surface of the Earth."  But in my class I always emphasize that the actual
acceleration of an object is due to the NET force acting ON that object
(Why don't the texts put that in for the F in F=ma?, i.e., F
sub(net)sup(on)) so that an object will have acceleration of magnitude 'g'
ONLY when the only force acting on it is gravity.  What I have noted is
that while 'g' is the magnitude of the Gravitational Field near the surface
of the Earth the Field itself is a Vector field (at least at the level we
use).  I've found that unless you continually emphasize the vector nature
of these things it gets lost!

Good Luck,

Herb Schulz
(herbs@interaccess.com)