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
Pulse wave velocity in the investigation of hypertension. The theory of the velocity of the transmission of the pulse through the circulation dates back to 1808 with the work of Thomas Young. The relationship between pulse wave velocity (PWV) and arterial wall stiffness can be derived from Newton's second law of motion (formula_1) applied to a small fluid element, where the force on the element equals the product of density (the mass per unit volume; formula_2) and the acceleration. The approach for calculating PWV is similar to the calculation of the speed of sound, formula_3, in a compressible fluid (e.g. air): formula_4, where