Re: [Phys-l] g...

[Jack Uretsky <jlu@hep.anl.gov>]

Hi John-
	What do you mean "units are the same"?  I agree that both grav 
mass, m_g and inert mass, m_i, are measured in kg.  Now define m_g = 
.9m_i, as I am free to do.  Then big G will be bigger by a factor of 
1/(.()^2.  The price I pay is that I have to be careful where I use m_g 
and m_i.
	The two quantities really paly quite different roles.  the m_g are 
the "charges" the define the strength of the gravitational field, the m_i 
measure the inertia that enters into Newton's laws.  Cavendish made the 
choice for us, when he chose to measure G using inertial masses as the 
strength of his charges.  Eotvos showed that the choice was sensible 
because it didnt's depend on the choice of material for the charges.
	Isn't your disagreement jsut esthetic?
					Regards,
						Jack


On Sun, 19 Nov 2006, John Mallinckrodt wrote:

> 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>
>
>
>
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