Published by Patrick Mutisya · 14 days ago
To understand that every physical quantity is expressed as a product of a numerical magnitude and a unit.
A physical quantity describes a property of the physical world that can be measured. It is represented mathematically as
\$Q = N \times U\$
where Q is the quantity, N is the numerical value (a pure number), and U is the unit that gives the quantity its meaning.
The International System of Units (SI) provides a consistent set of base units from which all other units are derived.
| Quantity | Symbol | SI Unit | Unit Symbol |
|---|---|---|---|
| Length | \$L\$ | metre | m |
| Mass | \$m\$ | kilogram | kg |
| Time | \$t\$ | second | s |
| Electric current | \$I\$ | ampere | A |
| Temperature | \$T\$ | kelvin | K |
| Amount of substance | \$n\$ | mole | mol |
| Luminous intensity | \$I_v\$ | candela | cd |
Derived quantities are formed by combining base quantities according to physical laws. Their units are products or quotients of base units.
| Derived Quantity | Symbol | Definition | SI Unit (symbol) |
|---|---|---|---|
| Velocity | \$v\$ | \$v = \dfrac{s}{t}\$ | metre per second (m s⁻¹) |
| Acceleration | \$a\$ | \$a = \dfrac{v}{t}\$ | metre per second squared (m s⁻²) |
| Force | \$F\$ | \$F = m a\$ | newton (N) |
| Energy | \$E\$ | \$E = F s\$ | joule (J) |
| Power | \$P\$ | \$P = \dfrac{E}{t}\$ | watt (W) |
When writing a measured quantity, the numerical value and unit must be together, e.g.,
Omitting the unit removes the physical meaning and can lead to serious errors (e.g., the Mars Climate Orbiter failure).