In physics and other sciences, energy
(from the Greek energos, meaning "active, working")[1]
is a scalar physical quantity, often represented by the
symbol E,[2] that is used to describe a conserved property
of objects and systems of objects. Several different forms,
such as kinetic, potential, thermal, electromagnetic,
chemical, nuclear, and mass have been defined to explain
all known natural phenomena.
Energy may be transformed from one form
to another, but it is never created or destroyed. 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.[3]
Although the total energy of a system
does not change with time, its value may depend on the
frame of reference. For example, a passenger in an airplane
has zero kinetic energy relative to the airplane, but
nonzero kinetic energy relative to the earth.
