coplanar forces

Equilibrium of rigid bodies acted on by coplanar forces.

A simple but elegant demonstration of the conditions of equilibrium for three or more coplanar forces acting either at a point, on a circular disc or on a rectangular shape.

In other words, "If many coplanar forces are acting on a body, then algebraic sum of moments of all the forces about a point in the plane of the forces is equal to the moment of their resultant about the same point."