moment of a force

In physics, measure of the turning effect, or torque, produced by a force acting on a body. It is equal to the product of the force and the perpendicular distance from its line of action to the point, or pivot, about which the body will turn. The turning force around the pivot is called the moment. Its unit is the newton metre.

The moment of a force can be worked out using the formula: moment = force applied × perpendicular distance from the pivot. If the magnitude of the force is F newtons and the perpendicular distance is d metres then:

moment = Fd

It is easier to undo a bolt using a long spanner than a short spanner. This is because more turning force is produced at the bolt (pivot) with less effort. A long spanner is an example of a force multiplier.

In a simple balanced see-saw, the forces acting on the left- and right-hand sides of the pivot are the same. This is known as balancing moments. Moving the load at one end will cause the see-saw to become unbalanced. To regain balance, the load on the opposite side must either be increased or its position changed. This is known as the principle of moments.

The application of balancing a moment of force (or a turning force) is used by a rope walker. By holding the pole in the middle the rope walker is balancing the turning force on either side of the rope. As the rope walker moves a leg, the turning force on one side is greater and becomes unbalanced. To regain the balance, the pole is shifted to balance the turning forces on both sides of the rope.

See also couple.