Tupac Yupanqui
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Relativistic mass
editTupac, there are two definitions of mass being used here. One is the invariant mass, which for single particles is the same as the rest mass. The other definition is the relativistic mass, which is NOT invariant, but changes with reference frame, same as total energy. People are always getting confused by the two kinds of mass and the two kinds of energy. Setting c=1, the rest mass (invariant mass) is the same as the rest/invariant energy. And the total energy is the same as the relativistic mass. But these 4 are equal to each other only when total momentum is zero, which means only in the COM frame. It is only there for systems that total energy is equal to invariant mass (and for particles, obviously total energy is rest mass). Anyway, see mass in special relativity for a full discussion of these 4 quantities, only 2 of which are invariant. SBHarris 03:56, 23 December 2009 (UTC)