In physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical energy is constant. If an object moves in the opposite direction of a conservative net force, the potential energy will increase; and if the speed of the object changes, the kinetic energy of the object also changes. In all real systems, however, nonconservative forces, such as frictional forces, will be present, but if they are of negligible magnitude, the mechanical energy changes little and its conservation is a useful approximation. In elastic collisions, the kinetic energy is conserved, but in inelastic collisions some mechanical energy may be converted into thermal energy. The equivalence between lost mechanical energy and an increase in temperature was discovered by James Prescott Joule.

भौतिकी में किसी यांत्रिक प्रणाली के किसी अवयव में निहित स्थितिज ऊर्जा तथा गतिज ऊर्जा के योग को यांत्रिक ऊर्जा कहते हैं। अर्थात यांत्रिक ऊर्जा किसी वस्तु की गति या उसकी स्थिति से सम्बन्धित है।