Re: [Phys-l] Weightless

[John Mallinckrodt <ajm@csupomona.edu>]

Bob,

In response to my pointing out that, since g is not equal to GM/r^2,  
one definition would have to be abandoned, you wrote:

> I still see both definitions in common practice in high school,  
> first year physics texts and in engineering texts.  So, apparently  
> neither has been abandoned yet.  Please let me know when you have  
> convinced the authors of these texts to change their definitions.

and in response to John Denker's pointing out that "horizontal" is  
not perpendicular to "toward the center of Earth" via an imaginary  
situation involving a pool contractor you wrote:

> If I were to hire a contractor to build a pool, I doubt that the  
> definition of weight would come into our conversation.  I certainly  
> would ask for references and visit pools they have already built.   
> Good luck with your quest.

In both cases I don't really understand the point of your seemingly  
dismissive comments.

Is it your position that there is something wrong with either of  
those points?  Is it, perhaps, that we should accept what  
introductory textbooks say about such matters?  More likely you  
simply find the discussion overly pedantic and/or some of the  
discussants including me arrogant.  If the latter, I regret it  
greatly, but I honestly don't feel the need to amend or apologize for  
anything I've written in this thread.

For my part, I've never taken any serious issue with textbooks that  
say that g = -GM/r^2 r_hat, because it's exactly correct in one  
important highly idealized situation (within the framework of  
Newtonian mechanics) and because it isn't a bad approximation in some  
more realistic situations.

I've always figured that those students who go on to become  
professional physicists will eventually make the necessary  
adjustments.  In other words, that they will come to appreciate the  
fact that gravitational forces *are* frame-dependent and are not  
generally determined by the masses and positions of nearby objects  
*despite* what introductory textbooks may say.

I've always assumed that practicing physicists will understand (or be  
easy to convince of) the deep significance and beauty of the  
principle of equivalence and appreciate the breathtaking simplicity  
of the world view that results from understanding that gravitational  
forces *are* inertial forces, that they are always given by mg where  
g is the local gravitational field, and where g is always easily and  
operationally determined by simply measuring the acceleration of a  
local freely falling object.

If that isn't the case, then I should probably revisit my relative  
lack of concern about what is taught in introductory textbooks,  
indeed, about what I myself teach.  Perhaps it *is* time 100 years  
after Einstein changed our view of gravity to start actually teaching  
that view in a much more systematic way.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>