Re: [Phys-l] Weightless

[John Denker <jsd@av8n.com>]

On 11/21/2006 06:50 PM, Robert Carlson wrote:

> Physics for Scientists and Engineers, Tipler, 3rd
> Edition page 84 discusses apparent weight and
> weightlessness as you have been using these terms. 
> Defining weight as mg and mMG/r^2 is common practice
> in high school and first year college level physics
> texts.  Engineering Statics and Dynamics texts also
> use these definitions for weight.  You are not alone
> in your views, at least at the level you are teaching.

Well, go ahead and teach it that way if you like, but for
the sake of the students, you should warn them that using
that definition will get them fired from a wide range of
blue-collar and white-collar jobs.

It leads to a wrong notion of horizontal, a wrong notion
of vertical, and a wrong value of |g|, at a level that is
unacceptable in applications such as architecture, surveying,
gunnery, and many others.

It is also inconsistent with the "operational" approach
that some people have strongly advocated, namely the idea
that weight is the force impressed on the scale under
standard conditions.

I doubt it would overburden typical students to say

  weight = mg       by definition
                [This is important in practice.]

  weight = GmM/r^2 + corrections that are less than 1% under
                     ordinary laboratory conditions
                 [*Any* connection of g to G is a fun physics
                 fact, with only limited importance in ordinary
                 terrestrial situations.]

 a) This is not unduly complicated.  If students can't handle this,
  they shouldn't be taking physics class at all.
 b) This is consistent with universally-accepted practical
  procedures for weighing things, and consistent with conventional
  notions of horizontal and vertical.
 c) This leaves the door open for a sensible discussion of
  weightlessness, which is, after all, the nominal topic of
  this thread.