Re: [Phys-l] g...

[John Mallinckrodt <ajm@csupomona.edu>]

On Nov 19, 2006, at 10:49 PM, Jack Uretsky wrote:

> Hi all-
>  	This "accident" is not really an accident.  The so-called
> equivalence principle is a definition.  We define Newton's constant  
> G so
> that the inertial mass for some element equals the gravitational mass.
> The "accident" is that when you do the definition for one element (or
> substance - however you want to define it) it holds true for all
> substancees, as demonstrated by the Eotvos experiments.  It is the  
> ratio
> of gravitational to inertial mass that is constant for all  
> substance (se
> Weinberg, Gravitation, Sec. I.2), and this ratio can be made unity  
> by a
> suitable definition of G (big G).
>  	This point seems to be missed in the discussion in Halliday &
> Resnick (3d Ed.) and, perhaps, in other elementary texts.  Feynman  
> doesnt
> stress the point in his Lectures, although, characteristically, he  
> sort of
> sneakss up on it.
>  				Regards,
>  					Jack

Jack,

I don't think it's right to call the equivalence principle a  
definition.  It is true that one might, in Newtonian mechanics, have  
measured "gravitational" and "inertial" mass in different units and  
then noticed that the ratio of gravitational to inertial mass appears  
to be the same for all substances.  But the equivalence principle  
doesn't tell us merely that we should measure these two different  
quantities in the same units, it says that there is only one  
quantity--mass--and that gravitational forces ARE inertial forces.

John Mallinckrodt

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

and

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