0580

Compound Measures

Number

Combining two quantities

  • A compound measure is built from two different measurements rather than one
  • It expresses a rate: how much one quantity changes for each unit of the other
  • The word "per" signals the division, and the units say which way round it goes
    • Speed measures distance per unit of time
    • Population density measures people per unit of area
    • Fuel consumption measures fuel used per unit of distance

Reading the formula from the units

  • The units of a compound measure state its formula, which removes any need to memorise them separately
  • A unit written as a/b means the quantity measured by a divided by the quantity measured by b
    • g/cm³ is grams divided by cubic centimetres, so it is mass divided by volume
    • people per km² is a count divided by an area