SI Units (AQA AS Physics): Revision Note

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 six 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

Derived units are derived from the seven SI base units

The base units of physical quantities such as the following can be deduced:

• Newtons, N

• Joules, J

• Pascals, Pa

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