Lesson 5 - Equilibrium and reactions

Equilibrium is a core concept of statics. Equilibrium of the body means that body is in the rest state relative to surrounding bodies. In other words, the body is static in some desired coordinate system. For instance, your house is in rest state (in equilibrium state) relative to the earth. The house isn't moving anywhere despite a set of forces is applied to him: gravity, background friction force, pressure of the fallen tree above the roof: