Welcome to Wikipedia. You may wish to take a look at the articles on invariant mass, special relativity, and mass in special relativity.
The (invariant) mass of a system of particles can be calculated from the general formula:
It's the same in all frames. But in the COM frame, the last term is obviously zero. Thus, IN THE COM frame, and this frame only, this mass, which is the invariant mass, is total energy/c^2 (or simply total E in natural units). However, it's not (in general) the sum of the rest masses of particles in the system. Unless they don't interact with each other and aren't moving with regard to each other.) But this is handy, inasmuch as the COM frame is the frame you weigh bound systems in, and thus their mass measured by weight is their total energy. In other frames the system invariant mass is the same, but the energy is different-- as is the case with single particles also. The system rules are just exentions of the rule for single particles: only in the rest frame is the particle energy the same as its invariant/rest mass. For systems, this is true only in the COM frame. That's all this article says. S B H arris 20:15, 24 October 2009 (UTC)
Welcome to Wikipedia. You may wish to take a look at the articles on invariant mass, special relativity, and mass in special relativity.
The (invariant) mass of a system of particles can be calculated from the general formula:
It's the same in all frames. But in the COM frame, the last term is obviously zero. Thus, IN THE COM frame, and this frame only, this mass, which is the invariant mass, is total energy/c^2 (or simply total E in natural units). However, it's not (in general) the sum of the rest masses of particles in the system. Unless they don't interact with each other and aren't moving with regard to each other.) But this is handy, inasmuch as the COM frame is the frame you weigh bound systems in, and thus their mass measured by weight is their total energy. In other frames the system invariant mass is the same, but the energy is different-- as is the case with single particles also. The system rules are just exentions of the rule for single particles: only in the rest frame is the particle energy the same as its invariant/rest mass. For systems, this is true only in the COM frame. That's all this article says. S B H arris 20:15, 24 October 2009 (UTC)