Energy is an abstract quantity of extreme usefulness in physics because it is defined in such a way that the total energy of any closed physical system is always constant (conservation of energy). It is impossible to overstate the importance of this concept in all branches of physics from elementary mechanics to general relativity. Energy is measured in units of mass times velocity squared, and the MKS and cgs units of energy are the Joule and erg, respectively. Other common units of energy include the Btu, calorie, and kilowatt hour.

The important quantity in physics known as work, which is the product of applied force over a distance, has units of energy. In fact, the notion that heat is a form of energy was one of the most important developments in classical physics and thermodynamics.

Energy is related to power P emitted over a time t by

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Btu, Calorie, Conservation of Energy, Energy Density, Energy Flux, Foot Pound, Heat, Joule, Kinetic Energy, Kilowatt Hour, Momentum Four-Vector, Potential Energy, Power, Ton of TNT, Work