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
Thermal velocity Boltzmann constant, formula_2 is the temperature, and formula_3 is the mass of a particle, then we can write the different thermal velocities: If formula_4 is defined as the root mean square of the velocity in any one dimension (i.e. any single direction), then If formula_4 is defined as the mean of the magnitude of the velocity in any one dimension (i.e. any single direction), then If formula_4 is defined as the most probable speed, then If formula_4 is defined as the root mean square of the total velocity (in three dimensions), then If formula_4 is defined as the mean of