The principle of moments, also called Varignon's theorem in mechanics, states that the moment of the resultant of forces acting at a common point equals the sum of
their moments about the same reference point (Baker and Haynes). If 
is the position of the point of application and 
, then
 |
This follows from the distributive property of the cross product and allows a force to be resolved into components before calculating its moment.
The term also describes the moment balance required for static equilibrium: the sum of the moments of all applied forces and couples is zero. This condition must be accompanied by zero resultant force; moment balance alone does not establish equilibrium. The mechanics theorem is distinct from the geometric Varignon's theorem concerning quadrilaterals.
