Re: [Phys-l] Mass and Energy

[John Denker <jsd@av8n.com>]

Fayngold, Moses wrote:

> > 2. Halliday and Resnick state "mass is simply another form of 
>
> energy".
>
>   In my view, this is highly negligent and totally misleading statement.
> What will a student learn from it, if understood literally? That, among
> varios forms of energy, there is one called mass! 

That is exactly what H&R meant to say ... literally and otherwise.

> Mathematically, this
> would mean that, among other things, at least for this form of energy
> (let us name it Ef) we would have just Ef = m in ANY system of units.

Any _consistent_ set of units.

> Which means that this form of energy does not add up to all others, for
> which (in conventional system of units) E = mc2, (where m is relativistic
> mass). 

There is a proverb that says no matter what you are doing, you can
always do it wrong.

If you measure force in pounds, mass in kg, and acceleration in
Gees, you're going to have trouble applying F=ma.  So don't do
it that way.

If you want to write E=m, that works just fine if you measure mass
in energy units ... which is absolutely routine in particle physics.
I happen to remember the mass of the electron in MeV but not in kg;
if you want it in SI units I'd have to do the conversion.

  (Alternatively, you could measure energy in mass units, but that
   is not nearly so commonly done.)

Real-world physics is full of conversions from one system of units
to another.
 -- Should molar entropy be measured in bits/particle or in joules/kelvin?
 -- Should mass be measured in kg or GeV?

It's really not worth starting a holy war over such issues.  Use
whatever's convenient, and do the conversions when necessary.