Non-concurrent Forces:
If non-concurrent coplanar forces are acting on a body, their resultant may be in the form of a couple in addition to a resultant force. If a body is to remain at rest after that there should neither be neither a resultant force nor a resultant moment, i.e. for equilibrium, we obtain:
Rx = ∑ Fx = 0
Ry = ∑ Fy = 0
M o = 0
Thus, in addition to the two conditions of equilibrium as in case of concurrent forces there is one more condition of equilibrium which can be stated in words as:
"The algebraic sum of moments of all of the forces w.r.t. any point in their plane must be equal to zero."