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Energy (from the Greek ?????e?a - energeia, "activity, operation", from ??e???? - energos, "active, working"[1]) is a scalar physical quantity, an attribute of objects and systems that is conserved in nature. In physics textbooks energy is often defined as the ability to do work or to generate heat. Several different forms of energy exist to explain all known natural phenomena. These forms include (but are not limited to) kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy. While one form of energy may be transformed to another, the total energy remains the same. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.[2] Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy relative to the earth.
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