International A LevelMaths: Mechanics 1EdexcelRevision Notes1. Mechanics Toolkit1.1 Quantities, Units & Modelling1.1.3 Derived Units

Derived Units (Edexcel International A Level Maths: Mechanics 1)

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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 ms-1 (Velocity is just speed with direction)
    • Acceleration = velocity ÷ time = ms-1 /s = m/s2 or ms-2
    • Weight or Force = mass x acceleration = kgms-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 Tip

  • 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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