Re: [Phys-l] mass or rest mass?

[John Denker <jsd@av8n.com>]

On 03/25/2011 01:34 PM, James McLean wrote:

> It was recently brought to my attention that numerous pages on wikipedia 
> are espousing "mass conservation."  See for example "Mass conservation", 
> "E=mc2", and "Mass and energy in special relativity".
>
> I wonder what everyone's reactions to these articles is.
>
> My impression is that the physics is essentially accurate, but that the 
> use of nomenclature is not so good.  

It's worse than that.  I looked at
  http://en.wikipedia.org/wiki/Conservation_of_mass

It's really quite wrong.  It mixes two ideas, neither of which is 
entirely correct:
 a) "The mass of a strictly closed system is constant"
     ... which is true physics, but is not properly called a 
     conservation law, and
 b) "The mass in a given region cannot change except insofar as
     mass flows across the boundary into adjacent regions" ...
     which is stated in the form of a conservation law but is
     only approximately true.

It must be emphasized that mass is only approximately conserved.  For
example:
 -- It is off by a few parts in 10^9 in ordinary chemical reactions.
 -- It is off by 100% in positronium annihilation under ordinary
    conditions (i.e. non-closed conditions).

It must be emphasized that conservation is not the same as constancy,
and conservation applies to non-closed systems.  For details, see
  http://www.av8n.com/physics/conservative-flow.htm

The problem with the conservation-of-mass wiki article is *not* 
just a problem with the nomenclature, because the second sentence 
of the article states "This is much like the conservation of energy 
...." which is either false or highly misleading (depending on how 
much "much" means to you).  In fact energy-conservation is exact, 
so far as we know, which puts it in a very different category from 
mass-conservation.

> It was my understanding that modern 
> usage in relativity is that "mass" means "rest mass", and thus that 
> "mass" and "energy" are not two names for the same thing 

Agreed.

In particular, mc^2 is the _rest energy_ of the particle, not the
total energy.

> (as one of those articles claims).
  e.g. http://en.wikipedia.org/wiki/Mass-energy_equivalence

Well, that's yet an additional category of problems.

That is not however the main problem with the conservation-of-mass
article, which claims to be "much like the conservation of energy"
(rather than trivially synonymous with conservation of energy).