Definition
Physical quantities are classified into two categories: base quantities and derived quantities.
Base quantities are chosen as independent quantities within a system of quantities. Derived quantities are defined from base quantities through mathematical relationships.
Base Quantities
Base quantities are not defined in terms of other physical quantities. They provide the foundation from which other quantities can be defined.
The seven base quantities are:
- Length
- Mass
- Time
- Electric current
- Thermodynamic temperature
- Amount of substance
- Luminous intensity
All other physical quantities can ultimately be expressed in terms of these base quantities.
Derived Quantities
Derived quantities are defined by combining base quantities mathematically.
A derived quantity may be defined directly from base quantities or from other derived quantities that can themselves be reduced to base quantities.
Unlike base quantities, derived quantities are unlimited in number.
Examples of Derived Quantities
| Derived Quantity | Defined As | Base Quantities Used |
| Area | Length × width | Length |
| Volume | Length × width × height | Length |
| Speed | Distance ÷ time | Length, Time |
| Velocity | Displacement ÷ time | Length, Time |
| Acceleration | Change in velocity ÷ time | Length, Time |
| Force | Mass × acceleration | Mass, Length, Time |
| Momentum | Mass × velocity | Mass, Length, Time |
| Density | Mass ÷ volume | Mass, Length |
| Pressure | Force ÷ area | Mass, Length, Time |
| Energy | Force × displacement | Mass, Length, Time |
| Power | Energy transferred ÷ time | Mass, Length, Time |
| Base Quantities | Derived Quantities |
| Chosen as independent quantities | Defined from base quantities |
| Not defined in terms of other quantities | Defined through mathematical relationships |
| Seven in number | Unlimited in number |
| Form the foundation of the system of quantities | Describe most measurable properties and physical relationships |
Key Points
- Physical quantities are classified as base or derived quantities.
- Base quantities are chosen as independent quantities.
- Seven base quantities form the foundation of the system of quantities.
- Derived quantities are defined from base quantities through mathematical relationships.
- A derived quantity may be defined directly from base quantities or through other derived quantities.
- Every derived quantity can ultimately be expressed in terms of base quantities.