Mass in special relativity what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference, The invariant mass is the ratio of four-momentum (the four-dimensional generalization of classical momentum) to four-velocity: and is also the ratio of four-acceleration to four-force when the rest mass is constant. The four-dimensional form of Newton's second law is: The relativistic expressions for "E" and "p" obey the relativistic energy–momentum relation: where the m is the rest mass, or the invariant mass for systems, and "E" is the total energy. The equation is
Distribution function (physics) plasmas are rarely in thermal equilibrium. The mathematical analogue of a distribution is a measure; the time evolution of a measure on a phase space is the topic of study in dynamical systems. 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