Moments

This lesson covers moments (turning effects of forces) as required by the Edexcel 9MA0 A-Level Mathematics specification. You need to be able to calculate moments about a point and apply the principle of moments to solve equilibrium problems involving rigid bodies.

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What is a Moment?

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

Moment = Force x Perpendicular distance from the point to the line of action of the force.

Moment of F about point P = F x d

where d is the perpendicular distance from P to the line of action of F.

Units: Newton-metres (N m).

Sign convention: Clockwise moments are usually taken as positive (or negative — just be consistent). In exam solutions, state your convention clearly.

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The Principle of Moments

For a body in equilibrium (not rotating):

Sum of clockwise moments about any point = Sum of anticlockwise moments about the same point

Equivalently: the net moment about any point is zero.