Re: [Phys-l] Weightless

[Robert Carlson <rcarlson@physicstoolkit.com>]

John,

I'm not sure where your intensity is coming from. 
What part of my post do you disagree with?  Do you
contest that Tipler discussed apparent weight and
weightlessness in a text with a copyright of 1991?  Do
you contest that it is common practice for weight to
be defined as mg and mMG/r^2 in high school and first
year college level physics texts?  Do you contest that
weight is defined this way in engineering texts such
as Hibbeler and Beer and Johnston?

Concerning the rest of your post, please name one
engineer you know of that has been fired because they
studied Hibbeler or Beer and Johnston and used their
definition of weight.

Bob Carlson

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From: John Denker <jsd@av8n.com> 
Date: Tue, 21 Nov 2006 19:52:33 -0500 

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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.