Equilibrium of Coplanar Forces:

Consider a rigid body acted upon by a number of coplanar forces. These forces shall cause the rigid body either

1. move in a particular direction without rotating

2. rotate at its own place without moving

3. rotate about itself and at the same time move in any particular direction, or

4. Remain totally at rest.

The last case is that of the state of equilibrium. In order to move the body in a specific direction there must presents a resultant force and to rotate there should exist a resultant couple. If the resultant force and the resultant couple both are absent, the body can neither move nor rotate. It shall remain at rest. Let us learn some typical cases.