Quantities and units in mechanics is Topic 6 of Paper 3 (Statistics and Mechanics), opening Section B ahead of kinematics, forces and moments. Year 1 covers the SI system mechanics is built on — length, time and mass as fundamental quantities, then derived ones such as velocity (m s⁻¹), acceleration (m s⁻²), force and weight (both N) — plus converting between units such as km h⁻¹ and m s⁻¹. Year 2 adds one more derived quantity, the moment, in newton-metres. On its own this strand is typically worth around 2 of the 300 marks, an estimate, since it's almost never set as a standalone question.
What that low mark count hides is how often the ideas resurface. A speed left in km h⁻¹, a mass forgotten in a moments equation, or a force term summed with a moment term all trace back to this topic, even though the mark loss shows up inside kinematics, forces or moments rather than here. There's no formula-booklet safety net either — the SI system and the definition of a moment are both expected from memory.
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: ≈2 marks — our estimate from the 2018–2025 papers, not an official weighting.
| Ref | Spec statement | Level |
|---|---|---|
| 6.1 | SI units and conversions | AS |
| 6.1 | Moment as a derived quantity | A2 |
Because a units question almost never stands alone, the mark loss for an unconverted speed or a missing length is recorded inside whichever bigger question it appears in — kinematics, forces or moments — as a method error, not a small deduction. Treat the SI system as infrastructure that has to be right before the mechanics starts.
A moments or forces equation that mixes a term in newtons with one in newton-metres, or drops a mass or length partway through, is treated as dimensionally incorrect and marked as a method error rather than an arithmetic slip. Check each term is the kind of quantity the equation needs before substituting numbers.
Weight, and anything calculated downstream of it, inherits mechanics' usual accuracy rule: 2 or 3 significant figures unless told otherwise, with more precise answers — including exact fractions — marked down, though an exact multiple of g is normally accepted. Decide the required accuracy before the final line.
Multiplying by 3.6 instead of dividing — or converting the distance part of a unit while leaving the time part alone — turns a plausible speed into one out by a factor of ten or more. Convert every quantity into SI base units first, and sanity-check whether the result is a realistic speed, mass or force.
Summing a term in newtons with one in newton-metres, or leaving out a perpendicular distance partway through, produces an equation that no longer makes physical sense — a method error, not a rounding slip. For an angled force, the moment needs the perpendicular distance, not the full length, so a sine or cosine usually appears in that term.
g = 9.8 m s⁻² is itself only a 2 significant-figure value, so a final weight or force quoted to four or five figures — or left as an exact fraction — overstates the precision the calculation supports. Carry extra decimal places through the working and round only the final line.
We haven’t published checked questions for this topic yet — a worked sample appears here only once a question has passed every check. In the meantime you can practise in the app.
The memorise list here is short but non-negotiable, since none of it is in the formula booklet: the fundamental quantities (length, time, mass), the derived ones built from them (velocity in m s⁻¹, acceleration in m s⁻², force and weight in newtons), and the moment as force times perpendicular distance, in newton-metres. Getting this automatic matters more than getting it right once, since it underpins every later calculation.
Build calculator and conversion fluency separately from any one mechanics question — practise the km h⁻¹ to m s⁻¹ conversion until dividing by 3.6 is automatic, and write the SI equation before evaluating it, so the method mark is protected even if a later number goes wrong. Decide the required accuracy, 2 or 3 significant figures unless stated otherwise, before the final line.
Since this topic is never examined alone, practise it inside the questions it actually appears in — a kinematics problem opening with a speed in km h⁻¹, a forces question with mixed units, or a moments question needing a perpendicular distance rather than a full length. Checking the units first protects marks throughout the rest of the paper.
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.