If two equal and opposite parallel forces (i.e. equal and unlike) are acting on a body, they don’t have any resultant force. That is no single force can replace two equal and opposite forces, whose line of action are different. Such a set of two equal and opposite forces, whose line of action are different, form a couple. Thus a couple is unable to produce any translatory motion (motion in a straight line). But a couple produce rotation in the body on which it acts.
Arm of Couple
The perpendicular distance (d) between the lines of action of the two equal and opposite parallel forces, is known as arm of couple.
