Arm of Couple:
Explain couple and Arm of couple?
Sol.: If the two equal and opposite parallel forces (that is equal and unlike) are acting on a body, they do not have any resultant force. That is no single force can replace the two equal and opposite forces, the line of action of which are different. Such a set of 2 equal and opposite forces, the line of action of which are different, form the couple.
Thus couple is unable to generate any translatory motion (motion in straight line). But couple produces rotation in body on which it is acting.
Arm of Couple
The perpendicular distance (d) between lines of action of two equal and opposite parallel forces, is called as arm of couple.