Speed
Speed describes how far something moves in a certain amount of time. In the International System of Units (SI), speed is measured in metres per second with the symbol \( \large \text{m/s} \).
SI unit of speed
The fundamental unit of speed is metres per second:
$$ \large 1\ \text{m/s} = \frac{1\ \text{metre}}{1\ \text{second}} $$
This means that an object moving at \( \large 1\ \text{m/s} \) travels 1 metre for every second that passes.
Derived units
In practice, several different units of speed are used depending on context and country:
| Name | Symbol | Relation to \( \text{m/s} \) | Note |
|---|---|---|---|
| Metres per second | m/s | \( \large 1\ \text{m/s} = 1\ \text{m/s} \) | SI unit |
| Kilometres per hour | km/h | \( \large 1\ \text{m/s} = 3.6\ \text{km/h} \) | Common in transportation |
| Miles per hour | mph | \( \large 1\ \text{mph} = 0.447\ \text{m/s} \) | Used in the UK and USA |
| Knot (nautical mile per hour) | kn | \( \large 1\ \text{kn} = 0.514\ \text{m/s} \) | Used in aviation and maritime contexts |
Use in different countries
Most countries use \( \large \text{km/h} \) (kilometres per hour) as the standard unit. English-speaking countries such as the UK and the USA use \( \large \text{mph} \) (miles per hour), while aviation and shipping use \( \large \text{kn} \) (knots).
Examples
- A person walking moves at about \( \large 1.4\ \text{m/s} \) or \( \large 5\ \text{km/h} \).
- A cyclist may travel around \( \large 25\ \text{km/h} \).
- A car on a motorway may drive \( \large 110\ \text{km/h} \), which equals \( \large 30.6\ \text{m/s} \).
- A passenger plane typically flies around \( \large 900\ \text{km/h} \) or \( \large 490\ \text{kn} \).