A LevelMathsEdexcelMechanicsRevision NotesQuantities & Units in MechanicsQuantities, Units & ModellingDerived Units

Derived Units (Edexcel A Level Maths): Revision Note

Exam code: 9MA0

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Edexcel Maths: MechanicsCambridge (CIE) Maths: MechanicsOCR Maths A: MechanicsAQA Maths: Mechanics

Derived units

What are derived units?

  • Derived units for quantities such as velocity, acceleration and force etc. are combinations of S.I. units

    • Speed or Velocity = distance ÷ time = m/s or m s-1 (Velocity is just speed with direction)

    • Acceleration = velocity ÷ time = ms-1 /s = m/s2 or m s-2

    • Weight or Force = mass x acceleration = kg m s-2 or N (N = Newtons)

1-1-3-diagram-1

Worked Example

a) Convert the following into S.I. units

i) 72 km h-2                    ii)   50 cm per minute                   iii)   14 g cm-3      

1-1-3-worked-solution-a

 

b) A cyclist takes 15 minutes to travel 2.54 km. Calculate the average speed in m s-1?

1-1-3-worked-solution-b

Examiner Tips and Tricks

  • Watch out for exam questions using non S.I. units such as feet and inches or miles per hour.

  • Make sure you are confident converting between these derived units,

  • Remember speed is velocity without direction and weight is a force measured in Newtons, not a mass measured in kg.

  • If you ever forget a formula, use the units it is measured in to figure it out,

  • e.g. m/s = metres ÷ seconds = distance ÷ time = velocity.

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