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
Mass flow rate These results are equivalent to the equation containing the dot product. Sometimes these equations are used to define the mass flow rate. Considering flow through porous media, a special quantity, superficial mass flow rate, can be introduced. It is related with superficial velocity, v, with the following relationship: The quantity can be used in particle Reynolds number or mass transfer coefficient calculation for fixed and fluidized bed systems. In the elementary form of the continuity equation for mass, in hydrodynamics: In elementary classical mechanics, mass flow rate is encountered when dealing with objects of variable mass, such as a rocket