Equilibrium of Concurrent and Coplanar Force System

The conditions of equilibrium for concurrent force systems are obtained by determining the equations that produce a zero resultant. The magnitude of the resultant of a concurrent force system is found by means of the equation:

Obviously, the resultant will be zero and equilibrium will exist when the following equations are satisfied:

These equations are known as the conditions of equilibrium. It is important to note that with two conditions of equilibrium, only two unknown quantities can be determined to create equilibrium of a concurrent force system. (from Engineering Mechanics, 2nd edition by Ferdinand Singer)

The presentation file below gives a thorough discussion on equilibrium of concurrent and coplanar force systems:
(switch to full screen mode for a better view)