Re: [Phys-l] g...

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

> > To me, the strange thing about the NIST definition is the terminology 
> > "local force of gravity".  Apparently "force of gravity" is NOT well-established!

> What's strange about that?  The physics requires it.  Force is
> a vector.  The vector direction is clearly different "here" versus
> the antipodes.  The vector magnitude is a function of altitude.

I'm not worried about directions, but rather about "force of gravity" vs "local force of gravity", which are defined as two different things by NIST.  And I'm thinking in Newtonian terms here, since that is the level of introductory texts.  To me, "the force of gravity" is the vector quantity with a magnitude 

F_grav = GmM/r^2.

As far as I can tell, this frame independent.  This equation does not depend on velocity or acceleration - just position.  

The term "local force of gravity" may be reasonable in relativity, but in a Newtonian perspective, it still seems odd and unnecessarily confusing.  Why invent a quantity called "local the force of gravity" which is "the force of gravity" - non-inertial frame effects? 

And even in relativity, wouldn't it be more consistent to talk about the local accelerations, not local forces?  




> > From NIST:
> > "The local force of gravity on a body, that is, its weight, consists of the 
> > resultant of all the gravitational forces acting on the body and the local 
> > centrifugal force due to the rotation of the celestial object."

> That makes sense to me.  That's the definition I've been using
> for a long time.

That also makes sense to me for a definition of WEIGHT, but I still wonder about also calling this the "local force of gravity".


 
> It also agrees with the convention of measuring altitude relative to
> sea level.  The sea is well approximated as an isopotential if and
> only if you include the centrifugal field as well as the GmM/r^2
> gravitational interaction.

That is thought-provoking....  Clearly the path that requires zero work is related to both gravity and centripetal terms.  Still, I would tend to define "true" gravitational equipotentials relative to a non-rotating frame.  Variations can be either 1) ignored to first order or 2) corrected for non-intertial terms and called something like "effective equipotentials" rather than "gravitational equipotentials".

Thoughts?  Again, I'm thinking in Newtonians terms.  Is this an unreasonable interpretation in a Newtonian world?  Do wwe need to introduce the relativistic interpretations in order to be "more correct"?  

Tim F