Articles by others on the same topic
Energy is a scalar quantity used to account for the capacity of a physical system to exchange work and heat and to change its state. Kinetic energy, potential energy and internal energy describe different contributions to one balance. Lifting a weight stores gravitational potential energy; allowing it to descend can turn that energy into motion, electrical output or heating.
Conservation of energy applies to the total accounting of an isolated system. A subsystem's energy can change because energy crosses its boundary, so a warming object or an accelerating cart does not violate conservation. For example, friction can reduce a sliding body's mechanical energy while increasing the internal energy of the contacting materials. In a closed dynamical description with time-translation symmetry, energy conservation is connected with that symmetry by Noether's theorem.
