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: