2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/87189An experimental formula, sometimes named as Newton-collision-formula, (v1-v2) = - e.(u1-u2) relating relative-velocities before & after impact of two bodies under linear-collision, is commonly used successfully for study of collision-dynamics. Although it seems possible as shown in text-books [1,2] to derive this relation, assuming (defining) e as ratio of impulses during restitution & deformation; but as yet, neither a good theoretical-basis nor a general-derivation of this formula based on obvious choice of energy-consideration, has been reported. In this brief-paper a relativistic-theoretical-basis is adopted and a general formula-derivation is given. It is shown, interestingly, that proof of the Newton-collision-formula is hidden in Einstein-special-relativity.Letter-to-editor type 2-page paper linking classical-physics to relativistic-physicsGeneral Physicstext