2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/128466We study physical situation considered by Einstein (Ann. Physik, 17, 1905) for his first derivation of mass-energy equivalence. Einstein introduced a constant $C$ in his derivation and reasoning surrounding $C$ and equations containing $C$ caused criticism by Ives. Here we clarify Einstein's derivation and obtain a value for constant $C$. The obtained zero value for $C$ suggests alternative explanation for Einstein's derivation and makes Ives's criticism a void and for which details are also presented in this paper.7 pagesHistory and Philosophy of PhysicsGeneral Physicstext