Moments is Topic 9 of Edexcel's 9MA0 Mechanics content, examined on Paper 3, and it's entirely second-year material with no equivalent in AS-level 8MA0. A moment is a turning effect — force times perpendicular distance from a pivot — and the topic puts a rigid body, usually a rod, beam or ladder, into equilibrium by resolving forces and taking moments together. It typically accounts for around 8 of the 300 marks, usually via one question per series.
Early problems keep every force parallel to a simple beam. Later ones move to non-parallel coplanar forces — typically a ladder against a rough wall or floor — where friction and one or two normal reactions join the picture, so a full force diagram matters before any equation is written.
There's no moments formula in the booklet — force × perpendicular distance is expected knowledge — and the two results it leans on most, weight = mg and F ≤ µR, are Appendix 1 facts to memorise.
The specification statements this topic covers. AS = Year-1 content, also assessed in the standalone AS course (8MA0); A2 = full A level only. Typical share of a 300-mark series: ≈8 marks — our estimate from the 2018–2025 papers, not an official weighting.
| Ref | Spec statement | Level |
|---|---|---|
| 9.1 | Moments and rigid-body equilibrium | A2 |
Examiners repeatedly report equations that mix a force term with a moment term, or drop a distance the diagram implies — both count as method errors. Check every term reads as force × perpendicular distance, and pick a pivot that removes an unwanted reaction rather than the easiest end to draw.
Non-parallel coplanar force problems commonly involve four or five forces — one or two weights, a floor reaction, a wall reaction, and friction at the foot. A diagram missing one is a frequent way marks disappear before any algebra starts, so label every force first.
Mechanics examiner reports flag a recurring mix-up between the reaction a support exerts and the resultant of the forces on a body — giving one where the question names the other loses the mark even with sound working otherwise.
Questions asking why a body stays in equilibrium, or is on the point of tilting or slipping, expect the full chain of reasoning finished with a conclusion, not just the opening link — stopping at 'friction acts along the floor' without linking back to equilibrium reads as incomplete.
It's easy to write an equation with some terms in newtons and others in newton-metres without noticing. Check each term individually — a moment is always force × distance, so anything missing its distance factor doesn't belong.
For a force acting at an angle — the weight of someone partway up a ladder — the moment needs the perpendicular distance to the line of action, usually a sine or cosine of the length along the body, not that length itself. Sketch the perpendicular first.
When a rod or ladder is drawn at an angle, the distance from the pivot to a given force changes with it — reusing a distance from an earlier, flatter sketch is a common source of error. Redraw the geometry at the stated angle before measuring from it.
g = 9.8 is itself only known to two significant figures, so an answer given to four or five figures, or as an exact fraction, is normally marked as over-accurate. Round to two or three significant figures unless an exact multiple of g is asked for.
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Because none of this topic's core facts are printed in the booklet, moment = force × perpendicular distance needs to be automatic, alongside weight = mg and F ≤ µR from Appendix 1 — the results it depends on most, all expected from memory.
With around 8 marks typically at stake per series, weight practice towards the harder non-parallel coplanar cases — ladder problems with friction and two reactions — over simple parallel-force questions, since that's where the difficulty concentrates. Get into the habit of drawing a complete force diagram before writing any equation, and choosing a pivot that removes an unknown reaction rather than defaulting to the same end each time.
Calculators are allowed throughout, but a 'show that' or 'using algebra' instruction still means the equation has to appear on the page rather than being solved silently — write it down first, and round the final answer to 2–3 significant figures once g has been used.
All 19 topics: Edexcel A level Maths topic guides. Reference: formula booklet vs memorise and grade boundaries.
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Original questions written for the Pearson Edexcel A Level Mathematics (9MA0) specification. Not affiliated with or endorsed by Pearson.