# Mass-gaining Particles?

 P: 204 Mass-gaining Particles? It doesn't "come from" anything. In my experience it's really not even all that common to talk about relativistic mass, we usually just write energy in terms of rest mass (which is invariant). It's a bit of a misleading concept anyway, because it implies (as you seem to have the impression) that it is related to some internal change in the object -- it's not. It more or less comes from the fact that mass-energy (rest mass, that is) can be written $E_{0} = mc^{2}$ For a particle moving relative to an observer, the energy of the particle would be measured by the observer to be $E = \gamma mc^{2}$ Thus you can write $m_{rel} = \frac{E}{c^2} = \gamma m$, where $m$ is the rest mass, equating $E_{0}$ with $E$ but again, it's kind of an ambiguous and misleading concept.