Re: Mass

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

> > How would we know how many atoms are in the sample?  By
> > measuring the mass?
>
> That would be pretty silly, wouldn't it?

Yes.

> > Volume?
>
> You could answer this yourself pretty easily:
>    http://www.google.com/search?q=mass+silicon+site%3Anist.gov
> which leads immediately to things like
>    http://physics.nist.gov/Divisions/Div842/Gp5/crystalc.html
> and
>    http://museum.nist.gov/exhibits/ex1/room6b.html

Yes, I suppose I could answer this myself pretty easily if I was smart
enough.  But I'm not so I hope you'll be a little patient with me.  I'm
asking how I would know that the mass of sample is 1 kg.  It just
appears to me that we don't count the number of atoms but rather the
concentration of atoms.  Am I wrong about that?  If not, it appears we
must use the volume to determine the mass, assuming we know the mass of
each atom (which we would know based upon the masses of the protons,
neutrons and electrons and the mass-equivalence of the binding energy).

But this can't be right, can it?  If so, why not just define 1 kg of
silicon in terms of the volume of the sample?  Why not use the mass of
each atom and the configuration of silicon to obtain the density and
then define 1 kg in terms of the volume and density of silicon?

[By the way, if a balance really measures inertial mass, it wouldn't
matter where the object's center of mass is, right?  Keep in mind that
the earth's gravitational field decreases with height.]

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301