Kinetic and potential energy is characteristics of interaction and motion of bodies and also their ability to make changes in the external environment. Kinetic energy can be defined for one body concerning another while potential always describes interaction of several objects and depends on distance between them.
Kinetic energy of a body call the physical quantity which is equal to a half of the work of body weight on its speed in a square. It is energy of the movement, it is equivalent to that work which force applied to a body at rest has to make to tell him assigned speed. After blow the kinetic energy can be transformed to other type of energy, for example, in sound, light or thermal.
The statement which is called the theorem of kinetic energy, says that its change is work of the net force applied to a body. This theorem is fair always even if the body moves under the influence of continuously changing force, and its direction does not coincide with the direction of its movement.
The potential energy is defined not by the speed, but mutual position of bodies, for example, concerning Earth. This concept can be entered only for those forces which work does not depend on a trajectory of the movement of a body, and is defined only its initial and final by provisions. Such forces are called conservative, their work is equal to zero if the body moves on the closed trajectory.
Conservative forces and potential energy
Gravity and elastic force are conservative, for them it is possible to enter a concept of potential energy. The physical sense is made by not potential energy, but its change when moving a body from one situation in another. The change of potential energy of a body in the field of gravity taken with the opposite sign is equal to work which is made by force for movement of a body. At elastic deformation the potential energy depends on interaction of parts of a body with each other. Possessing a certain stock of potential energy, the compressed or stretched spring can set a body which is attached to it in motion, that is to tell him kinetic energy. Besides elastic forces and weight property of conservatism other types of forces, for example, force of electrostatic interaction of charged bodies have. For friction force the concept of potential energy cannot be entered, its work will depend on the passable way.