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,
Distribution function (physics) Distribution function (physics) In molecular kinetic theory in physics, a particle's distribution function is a function of seven variables, formula_1, which gives the number of particles per unit volume in single-particle phase space. It is the number of particles per unit volume having approximately the velocity formula_2 near the position formula_3 and time formula_4. The usual normalization of the distribution function is where, "N" is the total number of particles, and "n" is the number density of particles – the number of particles per unit volume, or the density divided by the mass of individual particles. A distribution function may