Conservation of energy

The principle that the total energy of an isolated system remains constant, although energy can change form or move between its parts.

First law of thermodynamics: The thermodynamic principle that a system’s internal-energy change equals heat added minus work done by the system. It expresses energy conservation for systems that exchange heat and work.
Energy: A physical quantity that characterizes the capacity of a system to produce change or perform work. Conservation concerns the total of the different energy forms in a system.
Energy conservation in mechanics: The use of conserved mechanical energy to relate positions, speeds, and forces. It can solve motion problems without tracking acceleration at every instant.
Conservation of momentum: The principle that total momentum remains constant in an isolated system. It is a separate conservation law, with a different conserved quantity and symmetry.
Energy conservation in general relativity: The treatment of energy conservation in Einstein’s theory of spacetime and gravity. Curved spacetime permits local conservation while lacking a universal global energy in general.
Noether's theorem: A theorem connecting continuous symmetries of physical actions with conserved quantities. Time-translation symmetry yields energy conservation in many physical theories.
System boundary: A conceptual or physical surface separating a system from its surroundings. Whether energy is conserved depends on what the chosen system includes.
Roller coaster: A guided vehicle ride whose cars move along a track through changes in height and speed. The exchange between gravitational potential and kinetic energy predicts speed along a track.
Conservation of charge: The principle that electric charge is neither created nor destroyed in physical processes. Charge conservation tracks a distinct quantity, not energy.
Stress–energy tensor: A tensor describing energy density, momentum density, and stresses in relativity. Its covariant conservation expresses local energy-momentum balance in general relativity.

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