-When a body in motion impinges on another body at rest, which it is unable to move, it recoils, in order to continue its motion, and the angle made by the line of motion in the recoil and the plane of the body at rest, whereon the moving body has impinged, will be equal to the angle formed by the line of motion of incidence and the same plane.
Mugitzen den gorputz batek mugi-ezinezko beste bat, geldirik dagoena, jotzen duelarik, aitzina mugitzeko gisan isladatua da, eta jo duen gorputz geldiaren planoarekin isla-marrak osatzen duen angeluak, plano berarekin jotze-marrak osatzen duen angeluaren neurri bera du.