SI Units (Cambridge International A-Level Physics)

SI Base Quantities

Spec point — 1.2.1

• There is a seemingly endless number of units in physics

• These can all be reduced to seven base units from which every other unit can be derived

  • Only five of these are required for this course

• These seven units are referred to as the SI Base Units; this is the only system of measurement that is officially used in almost every country around the world

SI base units

Exam Hint You will only be required to use these five SI base units in this course, so make sure you know them!

Derived SI Units

• Derived units are non-SI units, such as joules or pascals

  • These derived units can all be written as combinations of the SI base units

• To deduce the combination of base units, it is necessary to use the definition of the quantity

• The newton (N), the unit of force, is defined by the equation:

  • Force = mass × acceleration

• Writing this equation in terms of the units of each quantity gives:

  • N = kg × m s^–2 = kg m s^–2
  • Therefore, the newton (N), written in terms of SI base units, is kg m s^–2

• The joule (J), the unit of energy, is defined by the equation:

  • energy = ½ × mass × velocity²
  • J = kg × (m s^–1)² = kg m² s^–2
  • Therefore, the joule (J) in SI base units is kg m²s^–2

• The pascal (Pa), the unit of pressure, is defined by the equation:

  • pressure = force ÷ area
  • Pa = N ÷ m² = (kg m s^–2) ÷ m² = kg m^–1 s^–2
  • Therefore, the pascal (Pa) in SI base units is kg m^–1s^–2