Whenever two forces of the same magnitude act in opposite directions along different lines, they have a zero resultant force, but do have a turning effect. In fact, the moment will be Fd about any point, where d is the perpendicular distance between the forces.

This is demonstrated in Figure 11.10.

Figure 11.10

In each of these situations:



Any set of forces like these with a zero resultant but a non-zero total moment is known as a couple. The effect of a couple on a rigid body is to cause rotation.