AQA A Level Physics复习笔记1.1.1 SI Units

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 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 Quantities Table


Exam Tip

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

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