Derived Units (Edexcel IAL Maths)

Spec point — 1.1

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)

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

Answer:

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

Answer:

Exam Hint

• 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.