A value that is equal to half the product of the mass of a given body and the velocity of this body squared is called the kinetic energy of the body or the energy of action in physics. A change or inconstancy of the kinetic or moving energy of the body over time will be equal to the work that has been completed in a given time by a certain force acting on the body. If the work of any force along a closed path of any type is equal to zero, then this kind of force is called a potential force. The work of such potential forces will not depend on the trajectory of the body. Such work is determined by the initial position of the body and its final position. The reference point or zero for potential energy can be chosen absolutely arbitrarily. The value, which will be equal to the work done by the potential force to move the body from a given position to the zero point, is called in physics the potential energy of the body or the energy of the state.
For different types of forces in physics, there are various formulas for calculating the potential or stationary energy of a body.
Work done by potential forces will be equal to a change in a given potential energy, which must be taken in the opposite sign.
If you add the kinetic and potential energy of the body, you get a value called the total mechanical energy of the body. When the system of several bodies is conservative, the law of conservation or constancy of mechanical energy is valid for it. The conservative system of bodies is such a system of bodies that is subject to the action of only those potential forces that are not dependent on time.
The law of conservation or constancy of mechanical energy is: "During any processes that occur in a certain system of bodies, its total mechanical energy always remains unchanged." Thus, the full or all mechanical energy of any body or any system of bodies remains constant if this system of bodies is conservative.
The mechanical energy of any system, which consists of n points continuously interacting with each other, is equal to the sum of the potential stationary and kinetic driving energy of the point system. If the system of these points is located in an external field in which conservative or constant forces act, then the total mechanical energy of this system is equated to the sum of the potential stationary and kinetic driving energy with the addition of the potential energy of this system in the external field. If there are also non-conservative forces in the system, then the total mechanical energy of the system cannot be conserved, but begins to decrease and its decrease is equal to the work of non-conservative forces in this system.
If only non-conservative forces act on a particle in the system, then the sum of potential and kinetic energy will be conserved. But at the same time interconversions of energies in the system are possible. Potential energy can turn into kinetic energy, and kinetic energy into potential energy.
The law of conservation or constancy of the full or all mechanical energy is always invariant, that is, its form of recording does not change, even when the initial point of reference is changed. This is a consequence of the law of the homogeneity of time.
When dissipative forces begin to act on the system, for example, such as the friction force, then the mechanical energy of this closed system gradually decreases or decreases. This process is called energy dissipation. A dissipative system is a system in which energy can decrease over time. During dissipation, the mechanical energy of the system is completely converted to another. This is fully consistent with the universal law of energy. Thus, in nature there are no completely conservative systems. Necessarily in any system of bodies or material points this or that dissipative force will take place.