🚀 go-pugleaf

palmer@sfu.ca (Leigh Palmer) writes:

> "Mass" as you use it
>is equal to neither inertial nor gravitational mass as those terms apply to
>any system more complicated than a particle at rest! 

Which only illustrates that those concepts are only really useful in
the context of Newtonian physics, IMHO.

To further confuse the
>beginning physics student you could explain that the mass of that lump of
>matter is independent of its temperature. 

Actually, neither definition of mass would be temperature-independent,
if you define it for the whole lump of coal.  When you raise the
temperature, the energy in the rest frame increases!  The sum of
the masses of all the particles is perhaps temperature-independent,
but as you complained before, that's not the mass of the lump.

>For the benefit of those too young to know I will point out that this con-
>vention has changed over the years. It caught on about 25 years ago and has
>been a problem ever since, especially conceptually. People actually believe
>that there is *scientific* justification for their positions regarding this
>convention. It is exactly as scientific as the observation that the bird we
>North Americans call a "robin" is not a *true* robin!

Here I do agree with you: the distinction really is semantic, and
the only real justifications for the new convention are pedagogical
or utilitarian.  I think that the new convention *should* be taught
to elementary physics students, along with a four-vector treatment
of SR when we teach them SR, but we must keep in mind that it is a 
convention used because it is more useful than the alternative, and
not a matter of physical fact.
 
>Actually the *only*
>justification for the current convention is that one would have to use the 
>term "rest mass" rather more frequently in particle contexts than the term 
>"relativistic mass". In earlier times both terms were used to minimize con-
>fusion.

One wouldn't use the term "relativistic mass" at all, actually.  It's
useless unless you must hang onto a formulation of relativity in
terms of three-dimensional quantities, and is of limited use even
there.  I do say "rest mass" when I want to be absolutely clear,
though.  There's no chance for ambiguity if you do.

-- 
Matt McIrvin   mcirvin@husc.harvard.edu   Long live short .sigs