Re: acceleration of gravity

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

QuistO writes:

> While we are on the subject of "pet peeves"
>
> 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.
> ...

I was wondering when this subject/topic would come up.  I agree with Quisto
here in that I prefer to consider "weight" to be the magnitude of any *non- 
gravitational* reaction force exerted on an object supporting it against 
gravity.  If there is no support there is no weight.  Objects in free fall
(and therefore in orbit) are by (this) definition weightless.  In order for
the supporting reaction force to be counted as sensible weight it should be
applied to the object as a "contact" force from the "bottom" (i.e. low
gravitational potential) surface of the object.  Here the meaning of the term
"contact" is loose enough so that a skydiver falling at terminal velocity has
his/her normal weight provided by the "contact" force of the air resistance
blowing up on the diver's underside (as seen in the frame falling with the
diver).

This definition of weight typically agrees with a student's previous naive,
and usually inarticulate, notion of weight.  However this concept of weight is
*more* complicated (and therefore more subtle to properly explain) than the
usual physics textbook definition of weight as just the magnitude of the
gravitataional force acting on a body.

> Obviously, we never want to do something that is obviously incorrect.

So we should only want to do something incorrect when it is not obvious that
it is incorrect?  How sneaky!

David Bowman
dbowman@gtc.georgetown.ky.us