The law of energy conservation is generalization of the skilled facts. Now it is considered the all-physical law which does not have any exceptions. According to it, energy is constant in size, does not appear and does not disappear, but only passes from one form into another.
In mechanics speak about two types of energy: kinetic and potential. Kinetic energy concerns the direct movement, potential indicates a possibility of the movement in the future. Potential energy — size conditional, depends on the chosen reference system. For an illustration it is possible to consider the mathematical pendulum. So call the ball suspended on a thread which makes not damped oscillations here and there. In extreme provisions it stops, but its potential energy at the same time is maximum. Passing through the center, the ball moves with the largest speed and has the maximum kinetic energy. Potential energy of a ball in median situation zero. In all points the sum of kinetic and potential energy of a ball — its general mechanical energy — remains to a constant. The mathematical pendulum — abstraction, ideal model. In case of the real physical pendulum, at a system there are friction forces and resistance of air. Fluctuations of a ball fade, and it seems that its energy decreases. Actually, it not so. Just mechanical energy partially passes into internal — in energy of the thermal movement of atoms and molecules. Friction forces and resistance are called dissipative forces (from English dissipate — to disseminate). They "disseminate" mechanical energy: thanks to them it gradually turns into internal, thermal. For certain you noticed more than once how at friction of a body heat up? Fire getting by friction is based on the same principle. So, at influence of dissipative forces, inelastic blows and other processes the body temperature increases. It is caused by increase in kinetic energy of atoms and molecules of a body in their chaotic movement. In process of development of science, it is required to enter new types of energy: electromagnetic, nuclear and so forth. The concept of energy, thus, continuously extends. The principle of conservation of energy pushes scientists in the direction of new researches. Any visible violation of this law says that there are phenomena which do not keep within a framework of the available scientific theories. Quite so was at discovery of radioactivity and a particle of a neutrino.