The more general invariant mass (calculated with a more complicated formula) loosely corresponds to the "rest mass" of a "system".
Thus, invariant mass is a natural unit of mass used for systems which are being viewed from their center of momentum frame (COM frame), as when any closed system (for example a bottle of hot gas) is weighed, which requires that the measurement be taken in the center of momentum frame where the system has no net momentum.
Under such circumstances the invariant mass is equal to the relativistic mass (discussed below), which is the total energy of the system divided by c^2 (the speed of light squared).
For other frames...
Thanks, Popeye, this is what I was referring to earlier when I said that the COM frame should be considered to be (and used as) a preferred frame in order to best understand the connection between rest mass and momentum. "Relativistic mass," with its infinite number of supposedly "equivalent" frames, just creates conceptual confusion.
Is leads GAHD to say that the universally accepted equation of E=Mc2 is just a "dumbed down layman's" formula, for example.
Well, then again, maybe it is other things which lead him to say that. It affords him another opportunity. to state that he aint dumb and that he aint no layman. He says he has an IQ that is...wait for it......over 140!!!!!! According to him, anyone who is "a few decimals short" of a 140 IQ aint nuthin but a damn foo.
I have to admit, though, that I've never seen another poster on a message board, when announcing his own IQ, give a number less than 200, so in that respect he's being quite modest, I suppose.
Edited by Moronium, 12 April 2019 - 08:19 AM.