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 "r" from the center of mass of the planet, whose mass is "M". Its initial speed is equal to its escape velocity, formula_6. At its final state, "f", it will be an infinite distance away from the planet, and its speed will be negligibly small and assumed to be 0. Kinetic energy "K" and gravitational potential energy "U" are the only types of energy that we will deal with, so by the conservation of energy, "K" = 0 because final velocity is zero, and "U" = 0 because its final distance is infinity, so where μ is the standard gravitational