Let us call the processes, in which occurs the transformation of mechanical motion into other forms of motion of matter, purely mechanical. System, in which there are purely mechanical processes, is called a conservative. This system is idealized, because it lacks friction and other resistances, resulting in scattering of mechanical energy, t. e. to its transformation into other forms of energy. In a conservative system may occur only to convert kinetic energy into potential energy and back. Work force, acting on a body in a conservative system, does not depend on the shape of the path, and is determined by only the initial and final positions of the body. Recall, what is an example of such forces is the force of gravity. It follows, in a conservative system the work force on closed path is zero.

 For such a system fair, the law of conservation of energy in the following form: in a closed conservative system the sum of kinetic and potential energies of all bodies, the components of the system, is constant. If we denote this sum E, in the absence of external influences on a conservative system

E=const         (6.7)

For example, with the free fall of a body the sum of its kinetic and potential energies remains constant.

Remember, that the only measure of transferring mechanical energy from one body to another is the work And. So, if the mechanical energy of a conservative system in any state is equal to E1, and then external forces make this system work And, the energy of the system increases by A and in new condition becomes equal to E2. Therefore, in this case

E2—E1=And.   (6.8)

Recall, in a broader sense, mechanical work And in all the phenomena of nature serves as the only measure of transmission and transformation of mechanical motion into other forms of motion of matter and returning.