Couple is a system of forces whose magnitude of the resultant is zero and yet has a moment sum. Geometrically, couple is composed of two equal forces that are parallel to each other and acting in opposite direction. The magnitude of the couple is given by
Where are the two forces and is the moment arm, or the perpendicular distance between the forces.
Couple is independent of the moment center, thus, the effect is unchanged in the following conditions.
- The couple is rotated through any angle in its plane.
- The couple is shifted to any other position in its plane.
- The couple is shifted to a parallel plane.
In a case where a system is composed entirely of couples in the same plane or parallel planes, the resultant is a couple whose magnitude is the algebraic sum of the original couples.