COUPLE (OF. cuplc, cople. Fr. couple, Sp. cdpula, front Lat. copula, bond, front co-, together apere, Gk. iirrap, haptein, to join). A couple is the name given in statics to a pair of equal parallel forces acting in opposite directions and at different points of a body. it is shown in the article MECHANICS (q.v.) that when two paral lel forces act in opposite directions on a body, they may be replaced by one equal to their dif ference, acting parallel to them and in the same plane with them, the point of application of this resultant being beyond the points where they are applied. This point recedes farther from the points of application of the original forces the nearer they approach equality, getting to an infinite distance when they become equal, and when their resultant accordingly is zero. In tnis limiting case, the forces constitute a couple. They have no tendency to translate the body; their action goes wholly to make it rotate about an axis parsing through its centre of inertia, and perpendicular to the plane in which the couple acts. Such being the case, a couple cannot be
replaced or counteracted by any single force, for such a. force would produce translation; it can only be replaced or balanced by other cou ples. The length of the straight line which meets the lines of action of the forces at right angles is called the 'arm' of the couple, and the product of the arm and either force is called the 'strength' or the 'moment' of the couple. It is evident that a couple can he replaced by one of equal strength. For a complete discussion of the composition and resolutions of couples, reference should be made to some treatise on statics, such as Routh, Treatise on .1natytica/ Statics (Cambridge, Eng., 1892) ; or Alinchin, Statics (New York, 1892). See AlEcnAstes.