A pair of two equal and unlike parallel forces (i.e, forces equal in magnitude with lines of action parallel to each other and acting in opposite directions) is known as a couple.
A couple is unable to produce any translatory motion (i.e, motion in a straight line). But it produces a motion of rotation in the body, on which it acts. The simplest example of a couple is the forces applied to the key of a lock while locking or unlocking it.
Characteristics of a Couple:
A couple (whether clockwise or anticlockwise) has the following characteristics:
1. The algebraic sum of the forces, constituting the couple is zero.
2. The algebraic sum of the moments of the forces, constituting the couple about any point is the same and equal to the moment of the couple itself.
3. A couple can’t be balanced by a single force. But it can be balanced only by a couple of opposite senses.
4. Any no. of coplanar couples can be reduced to a single couple, whose magnitude will be equal to the algebraic sum of the moments of all the couples.