Re: [Phys-L] foundations of physics: Galilean relativity, including KE

[John Denker <jsd@av8n.com>]

On 09/30/2015 12:48 PM, Jeffrey Schnick wrote:
> Can we agree that the total internal energy of a system is the mass
> of that system?

I agree with the intended meaning, I agree with
the physics idea ... but the terminology is tricky.
It might be safer to say the /rest energy/ is equal
to the mass of the system, in the appropriate units.
That's pretty much a direct translation of Einstein's
equation into words.

	E_0 = m c^2

I suggest calling the LHS the /rest energy/ because 
thermodynamics books have various definitions of 
"internal energy" that you probably wouldn't be 
very happy with.

Also I suggest writing Einstein's equation with 
a subscript zero on the E, to remind people it's 
just one piece of the energy.

The idea of /rest/ energy refers to the CM of the
system being at rest.  If you have a spinning 
flywheel, or thermal phonons in a solid, the KE
of the excitations /relative to the CM/ is part
of the system's rest energy and contributes to
the mass of the system.