Moments

The topic on one screen

• The moment of a force about a point is $F\times d$, where $d$ is the perpendicular distance from the point to the force's line of action. Its unit is $\text{N m}$.
• For a rigid body in equilibrium, the resultant force is zero and the resultant moment about any point is zero. Force equilibrium alone does not prevent rotation.
• Choose the moment centre to eliminate the most unknown forces. Reactions whose lines of action pass through that point contribute zero moment.
• A uniform rod or lamina has its centre of mass at its geometric centre. For a non-uniform body, use the stated or deduced centre of mass rather than the midpoint.
• For a rod at angle $\theta$ to the horizontal, a vertical force has horizontal lever arm such as $x\cos\theta$, while a horizontal force has vertical lever arm such as $x\sin\theta$.
• At the point of tipping, the reaction at the support that is losing contact is zero and the body is about to rotate around the remaining contact point.
• After taking moments, still resolve horizontally and vertically to find remaining reactions or friction. Keep force units and distance units consistent throughout.

Where students actually lose marks

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Questions are written in the style of past Edexcel papers (source shown on each) — never copied from them.

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