The first condition of equilibrium:

                    A body is said to satisfy the first condition of equilibrium if the resultant of all the forces acting on it is zero.

Explanation:

Let n number of forces F₁, F₂, F₃,…………., F_n is acting on a body such that

                           F₁+F₂+F₃+……….+F_n  = 0

Or 

∑ F= 0……………. (1)

The symbol ∑ is a Greek letter called Sigma used for summation. Equation (1)

Is called the first condition of equilibrium.

The first condition of equilibrium can also be stated in terms of the x and y components.

F_{1x}+F_{2x}+F_{3x}.....+F_{nx} =0

And 

F_{1y}+F_{2y}+F_{3y}.....+F_{ny} =0

Or

∑ Fx = 0

And 

∑ Fy  = 0

Examples:

A book lying on the table or a picture hanging on the wall, is at rest and thus satisfies the first condition of equilibrium and is thus in equilibrium.

   A paratrooper coming down with terminal velocity (constant velocity) also satisfies the first condition for equilibrium and is thus in equilibrium.