Conservation laws in mechanics are formulated for the closed systems which are often called still isolated. In them bodies are not affected by external forces, in other words, there is no interaction with Wednesday.
Law of conservation of momentum
Impulse call a measure of mechanical motion. Its application is admissible when it is transferred from one body to another without transformation into other forms of motion of matter.
In interaction of bodies the impulse of each of them can be transferred in whole or in part to another. At the same time the geometrical sum of impulses of all bodies making the closed isolated system remains to a constant whatever interaction conditions were. This statement in mechanics is called law of conservation of momentum, it is a direct consequence of the second and third laws of Newton.
Law of conservation and transformation of energy
Energy is the general measure of all types of the movement of matter. If bodies are in the closed mechanical system, at the same time they interact among themselves only by means of elastic forces and inclination, then work of these forces is equal to change of potential energy which undertakes with the opposite sign. At the same time the theorem of kinetic energy says that work is equal to change of kinetic energy. From this it is possible to draw a conclusion that the sum of kinetic and potential energy of bodies which make the closed system and interact among themselves only by means of elastic forces and inclination, it is invariable. This statement is called the law of energy conservation in mechanical processes. It is carried out only if in the isolated system of a body work at each other with conservative forces for which the concept of potential energy can be entered. Friction force is not conservative as its work depends on length of the passable way. If she acts in the isolated system, mechanical energy does not remain, its part passes into internal, for example, there is a heating. Energy does not arise and does not disappear in any physical interactions, it only turns from one form into another. This fact expresses one of fundamental laws of the nature — the law of conservation and transformation of energy. The statement that it is impossible to create the perpetual motion machine - the machine which is capable to make work unlimited time is its consequence, without spending energy. The unity of matter and the movement found the most general reflection in Einstein's formula: ΔE=Δmc^2, where ΔE – energy change, with - the speed of light in a vacuum. According to it increase or reduction of energy (number of the movement) leads to change of weight (amounts of matter).