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
Escape velocity alternative expression for the escape velocity formula_6 particularly useful at the surface on the body is: where "r" is the distance between the center of the body and the point at which escape velocity is being calculated and "g" is the gravitational acceleration at that distance (i.e., the surface gravity). For a body with a spherically-symmetric distribution of mass, the escape velocity formula_6 from the surface is proportional to the radius assuming constant density, and proportional to the square root of the average density ρ. where formula_14 The escape velocity "relative to the surface" of a rotating body depends on