SI Units (OCR A Level Physics)

SI Base Quantities

• There is a seemingly endless number of units in Physics

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

• These six 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 Quantities Table

Derived Units

• Derived units are derived from the seven SI Base units

• The base units of physical quantities such as:

  • Newtons, N

  • Joules, J

  • Pascals, Pa, can be deduced

• To deduce the 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

  • N = kg × m s^–2 = kg m s^–2

  • Therefore, the Newton (N) in 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