Balance is the state of rest of the body. In a general sense, balance implies the balance of the body, ie the ability of the body to remain at rest or in motion. We say that the body is in balance if it does not change its position in relation to the reference body, ie. it does not move in relation to it. Equilibrium occurs in different systems, according to which there are different types of equilibrium. Some of the most common types of equilibrium are: mechanical equilibrium, thermodynamic equilibrium, thermal equilibrium and hydrostatic equilibrium.
Mechanical equilibrium includes static and dynamic equilibrium.
Static balance means the balance of the body at rest. The condition of static balance is: In order for the body to be at rest, it must not perform translational or rotational movement. The body cannot move translationally if the sum of all the forces acting on it is equal to zero, however, the body can then perform a rotational movement. In order for the body to be in a state of equilibrium, it is necessary that the sum of all forces and moments of forces acting on it be equal to zero.
Dynamic balance means the balance of the body in a state of movement. In order for a body to be in a state of dynamic equilibrium, it is necessary that the sum of all forces acting at individual, material points of that body be equal to zero. The principle of dynamic equilibrium is a different formulation of the D'Alembert principle. Thus, the following also applies to dynamic equilibrium: During the movement of a material point, the force of inertia is in equilibrium with all forces acting on the material point. That is, in the system of material points, which is in motion, and the forces of inertia form a system of forces that is in equilibrium. The role and significance of the D'Alembert principle is great, because it enables dynamic problems to be solved by static methods.
The basic feature of mechanical balance is stability. There is a stable, unstable (unstable) and indifferent balance. The body is in a state of stable equilibrium when it is in such circumstances that when it is at least slightly removed from the equilibrium position, it shows a spontaneous tendency to return to its original equilibrium position. Unstable or unstable balance has bodies in which with a small movement there is a tendency to move even further from the original