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Consider a light rod AB capable of turning about a point O. Two equal forces each of magnitude F are acting at A and B in opposite directions. Moment of the forces about O = F × OA + F × OB = F(OA + OB) =  F × AB. 

Thus the moment of a couple called the torque is independent of the point about which it is capable of turning. The moment of a couple (torque) is equal to the product of one of the forces and the perpendicular distance between the lines of action of the forces.