SI Units (Cambridge (CIE) A Level Physics)

SI base quantities

• 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

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 m² s^–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^–1 s^–2