Mass in special relativity (including the kinetic energy of the gas) only when it is measured in the center of momentum frame. Just as is the case for single particles, the calculated "rest mass" of such a container of gas does not change when it is in motion, although its "relativistic mass" does change. The container may even be subjected to a force which gives it an overall velocity, or else (equivalently) it may be viewed from an inertial frame in which it has an overall velocity (that is, technically, a frame in which its center of mass has a velocity). In this case,
Atomic mass constant Atomic mass constant In physics and chemistry, the atomic mass constant, "m", is one twelfth of the mass of an unbound atom of carbon-12 at rest and in its ground state. It serves to define the atomic mass unit and is, by definition, equal to 1 u. It is inverse of Avogadro constant (1/N) when expressed in grams (instead of SI unit kilogram). The CODATA recommended value is In practice, the atomic mass constant is determined as the ratio of the electron rest mass "m" to the electron relative atomic mass "A"(e) (that is, the mass of the electron on