BunnyBaby

Hi

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:

$\left(Wc^{2}\right)^{2}=\left(\sum E\right)^{2}-\left\|\sum \mathbf {p} c\right\|^{2}$

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. SBHarris 20:15, 24 October 2009 (UTC)

Thanks for the welcome. Oh, ok my bad. I've never heard of this definition before, and I'm a physics grad student, but I see that you linked to it which is good as it will help avoid further confusion. I apologize for being so trigger happy with the delete key. BunnyBaby (talk) 22:32, 24 October 2009 (UTC)

Which hadn't you heard of? Invariant mass is very useful. The invariant mass for any system is just its summed 4-momentum divided by c (actually the 4-velocity to get rid of the vectors). SBHarris 19:54, 26 October 2009 (UTC)

I hadn't heard about invariant mass of a system BunnyBaby (talk) 23:30, 26 October 2009 (UTC)

Ah. This is one of the more common ways to use it. Calculate the invariant mass of 2 or 3 particle decay product system, and that's the rest mass of the parent particle (no matter what its kinetic energy was). The parent often having a half-life too short for it to be seen directly anyway. SBHarris 01:33, 27 October 2009 (UTC)