Moment of force

When forces are applied to a rigid body, they may have a translational or a rotational effect or both. When you push or pull on furniture to move it across a room, your force has a translational effect. When you push on a door to open it, your force has a rotational effect. Some doors have springs to keep them closed. If you push on the middle of such a door, it is much more difficult to open it than if you push it on the side furthest away from the hinge. Thus the force must have a greater turning effect the further it is away from the hinge. In fact, it can be shown by experiment that the turning effect of a force is directly proportional to the perpendicular distance of the force from the point about which the turning is to take place. As we might expect, the turning effect is also proportional to the magnitude of the force.

We can now define a measure of the turning effect of a force. It is called the moment of the force and is equal to the magnitude of the force multiplied by its perpendicular distance from the point about which the turning effect is being measured. A turning moment is also called a torque and since it is measured as force times distance, its SI unit of measurement is the newton metre (N m).