The turning effect of a force is called the moment of the force (or sometimes torque).

Not all objects rotate about a pivot, they may turn about a hinge or a fulcrum etc. The general name axis of rotation  applies to all cases. This name emphasises the fact that rotation does not take place about a point but about a line. The line (axis) is perpendicular to the plane in which the forces act.

The Unit of Moment

The magnitude of the moment of a force F, acting at a perpendicular distance from the axof rotation, is given by F x d

The unit in which it is measured is the newton metre, N m.

 It may appear at first sight that this unit could apply in another context, as work done by a force in moving a particle through a linear distance is also product of force and distance, suggesting the Newton metre as the unit.

However, as we always use the joule as the unit of work there is no confusion  over the N m which is used exclusively for moments.

The Sign of a Moment

Earlier, when we were collecting components of forces, we chose a positive direction ; components in that direction had a + sign while components in the opposite direction took a negative sign.

In the same way, we choose a positive sense of rotation when dealing with a system of moments. If, for example, we decide to make anticlockwise the' positive sense, an anticlockwise moment has a + sign while a clockwise moment has a - sign. The resultant moment of a number of forces is then the algebraic sum of the separate moments.

The positive sense does not always have to be anticlockwise; an individual choice can be made for each problem.

Zero Moment

When a force passes through the axis of rotation, its distance from that axis is zero. Therefore the moment of the force about that axis is zero.

Determining the Sense of Rotation

Most people looking at a diagram can see immediately the sense of rotation that a particular force would cause. From experience however we know that there are a few who have a `blind spot' here. There is a simple ploy for any readers who have this problem:

stick a pin into the point on the diagram about which turning will take place and pull the page ( gently! ) in the direction of the force. You will then see the rotation happening.