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Edexcel A-level Maths revision notes

M6 Quantities and units in mechanics — notes on one page

Every Edexcel A-level Maths 9MA0 specification point in M6, read straight through: 1 point with definitions, explanations, mark-scheme wording and worked examples. Practice questions are in the printable question pack.

Checked against Edexcel 9MA0 section M6. Review basis: the qualification registry sourced from the Pearson Edexcel Level 3 Advanced GCE in Mathematics (9MA0) specification; registry verification recorded 11 July 2026.

How this checking works

M6.1 · Understand and use fundamental quantities and units in the S.I. system: length, time, mass; understand and use derived quantities and units: velocity, acceleration, force, weight, moment.

  • The fundamental S.I. quantities are length in metres, time in seconds and mass in kilograms; velocity uses m s−1\text{m s}^{-1}, acceleration m s−2\text{m s}^{-2}, force and weight newtons, and moment N m\text{N m}.
  • Convert all measurements to consistent S.I. units before substituting, using for example 1 km h−1=518 m s−11\,\text{km h}^{-1}=\dfrac{5}{18}\,\text{m s}^{-1} and 1 tonne=1000 kg1\,\text{tonne}=1000\,\text{kg}.
  • Since F=maF=ma, 1 N=1 kg m s−21\,\text{N}=1\,\text{kg m s}^{-2}; multiplying a force by a perpendicular distance gives a moment in N m\text{N m}.
  • A common error is to confuse mass with weight: mass is measured in kilograms, whereas weight is a force measured in newtons; a moment is not measured in newtons alone.

Worked example

A resultant force gives a 750 kg750\,\text{kg} car an acceleration of 1.6 m s−21.6\,\text{m s}^{-2}. Find the force in newtons and write the newton in fundamental S.I. units.

  1. 1.Using F=maF=ma, F=750×1.6=1200 NF=750\times1.6=1200\,\text{N}.
  2. 2.Since mass has unit kg\text{kg} and acceleration has unit m s−2\text{m s}^{-2}, the fundamental-unit form is kg m s−2\text{kg m s}^{-2}.

Answer: 1200 N1200\,\text{N}; 1 N=1 kg m s−21\,\text{N}=1\,\text{kg m s}^{-2}

Common mistakes

  • Don't convert centimetres to metres by dividing by 1010 rather than by 100100.
  • Don't treat the newton as a fundamental unit rather than deriving it from mass times acceleration.

Exam tip. Carry units through the calculation and express derived force units as kilograms metres per second squared when requested.