# Relativistic momentum and mass [duplicate]

Is there even a relativistic mass or just relativistic momentum? How does one reason to prefer one over another? What is the problem with saying a hot gas will have more mass/inertia to it?

## marked as duplicate by jinawee, Emilio Pisanty, Brandon Enright, Dan, Kyle KanosJan 13 '14 at 22:40

Relativistic mass is not a useful concept. It is the same thing as total energy, just in different units. Relativistic momentum on the other hand is useful, because there is no such duplicity. An object composed of fast moving constituents does have higher rest mass, because there is more energy in this object. But you don't need relativistic mass to calculate the rest mass of such object. Total energy is enough.

• By relativistic mass I mean, $m/ \gamma$ – user37026 Jan 13 '14 at 11:40
• @L-L Yes, me too. This concept of relativistic mass is basically identical with total energy and therefore is not useful. – mpv Jan 13 '14 at 11:54
• @L-L did you mean to write $m \times \gamma$? If not then I don't think mpv understood what you meant. – JeffDror Jan 13 '14 at 13:03
• @JeffDror Good point. I overlooked the incorrect expression. – mpv Jan 13 '14 at 14:37