- Moments are about the turning effect produced by a force around a pivot point. The further away the force is to the pivot point, the greater the turning effect. As this is linear, the moment of a force about a point can calculated as the product of the magnitude of the force and the distance from the pivot point.
If the distance isn’t directly given, it can be worked out with some trigonometry magic.
The overall turning effect on a body can be calculated as the sum of all the moments about a point choosing clockwise or anti-clockwise as a positive direction.
- In the example above:
6×17 + 4×11 - (3×10 + 1×9) = 107 Nm Clockwise
Problems relating to bodies in equilibrium can be solved by equating the clockwise and anti-clockwise moments about a point, given that the body will not be moving. It can also sometimes be helpful to resolve the forces horizontally, especially in scenarios where there is a rod resting on supports.
A uniform rod can be considered as having its weight acting from its centre.