If you have two forces (for instance, a couple of forces) acting on an object and the forces are: parallel in opposite directions of equal size not along the same line of action ... you have got a couple. Couples produce rotation only. So the object spins faster and faster but it remains on the spot. (In physics speak - no linear acceleration or translation.) How can we calculate the moment (or torque) of a couple? /**/ So to calculate the moment (or torque) due to a couple you can use: Moment = Magnitude of one force x perpendicular distance between the lines of action of the forces. Have a go at this question: /**/