This page covers Moments, Couples and Equilibrium.

The moment of a force about a point is found by multiplying the magnitude of the force by the perpendicular distance from the point to the line of action of the force.


  • The moment of the force F about point A is F × d.

Moments can be either clockwise or anti-clockwise. In the above example, the moment is clockwise (because the force would cause a body to move in a clockwise direction relative to A).

The moment of a force about a point measures the turning effect of the force about that point.


A couple is a system of forces which has a net moment but does not have a resultant force in any one direction.

Here is a very simple couple:


In any direction, the resultant force is zero but the moment about A is clearly not zero. Such forces acting on a body might cause the body to rotate, but not move in any one direction.

If a system is in equilibrium, the system will have zero resultant force and the sum of the moments about any point will be zero.


The London Interdisciplinary School banner
Pass Your GCSE Maths Banner