There are two kinds of **mechanical energy**; * kinetic energy* point of the body and the potential energy of the system. Mechanical energy of a system of bodies is equal to the sum of the kinetic energies of the system of bodies and potential energies of their interaction:

*Mechanical energy = Kinetic energy + Potential energy*

## The law of conservation of mechanical energy is important:

In the inertial frame of reference, the mechanical energy of the system remains constant (unchanged, preserved), provided that the work of internal friction forces and the work of external forces over the bodies of the system is zero (or so small that they can be neglected).

### Kinetic Energy

As one of the types of **mechanical energy**, the kinetic energy of a point body is equal to the work that the body can do over other bodies by reducing its speed to zero. In this case, we are talking about inertial reference systems (ISO).

The kinetic energy of a point body is calculated by the formula K = (mv2) / 2.

The kinetic energy of the body increases when a positive work is done on it. And it increases by the amount of this work. When making a negative work on the body of its kinetic energy is reduced by an amount equal to the module of this work. The conservation of kinetic energy (the absence of its changes) says that the work done on the body was zero.

### Potential Energy

Potential energy is a type of **mechanical energy** that can be possessed only by systems of bodies or bodies considered as systems of parts, but not by a single point body. The potential energy of different systems is calculated differently.

Often considered a system of bodies is “body – Earth” when a body is near the surface of the planet (in this case, the Earth) and is attracted to it by gravity. In this case, the potential energy is equal to the work of gravity when the body is lowered to zero height (h = 0):

*P = mgh*

The potential energy of the “body – Earth” system is reduced when performing positive work by gravity. This reduces the height (h) of the body above the Ground. With increasing height, the gravity performs negative work, and the potential energy of the system increases. If the height does not change, the potential energy is saved.