3.19 Using Units Correctly

SI Base Units

• Every time you measure or calculate a quantity, you need to give its units
• All units in Physics can be reduced to six base units from which every other unit can be derived
  • These other quantities are called derived units
• 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

Derived Units

• Derived units are derived from the seven SI Base units mathematically
• 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

• It is essential that the correct scientific measurements are used when discussing experiments in physics
• Ensure that the correct symbols are used in conjunction with the unit of measurement
  • E.g. m³ for cubic metres

Units of Measurement Table

• Note:
  • cm³ is the same as millilitre (ml)
  • dm³ is the same as litre (l)