Derived Units (Edexcel A Level Maths) : Revision Note
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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 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)
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
b) A cyclist takes 15 minutes to travel 2.54 km. Calculate the average speed in m s-1?
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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