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.