Under what conditions do free mechanical vibrations occur?

Mechanical vibrations occur when the following conditions are met:
1. In one specific position of the body in space, called the position of equilibrium, the resultant of the forces applied to the body must be equal to zero. When removing the body from the equilibrium position, the resultant of all forces must be different from zero and directed to the equilibrium position.
2. For the beginning of free oscillations, the system must be brought out of the equilibrium position by external interaction.
3. The forces of friction in the system should be small compared to the forces that depend on the coordinates