2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/87955The k-calculus was advocated by Hermann Bondi as a means of explaining special relativity using only GCSE level mathematics and ideas. We review the central derivations, using proofs which are only a little more elegant than those in Bondi's books, and extend his development to include the scalar product and the mass shell condition. As used by Bondi, k is the Doppler red shift, and we extend the k calculus to include the gravitational red shift and give a derivation of Newton's law of gravity using only 'A' level calculus and basic quantum mechanics.9 pages, 12 diagramsGeneral Physicstext