In the
physical sciences,
mechanical energy is the sum of
potential energy and
kinetic energy. It is the
energy associated with the motion and position of an object. The principle of conservation of mechanical energy states that in an isolated system that is only subject to
conservative forces the mechanical energy is constant. If an object is moved in the opposite direction of a conservative net force, the potential energy will increase and if the
speed (not the
velocity) of the object is changed, the kinetic energy of the object is changed as well. In all real systems, however,
non-conservative forces, like
frictional forces, will be present, but often they are of negligible values and the mechanical energy's being constant can therefore be a useful approximation. In
elastic collisions, the mechanical energy is conserved but in
inelastic collisions, some mechanical energy is converted into heat. The equivalence between lost mechanical energy (
dissipation) and an increase in
temperature was discovered by
James Prescott Joule.