Topic 11. SI Units
The International System (SI) of units defines seven base units. These include:
On its own, each base unit describes a particular quantity — distance, mass, time and current. We also combine these quantities to describe other quantities. If a model plane flies a distance of 1 metre in 1 second, we say its velocity is 1 metre per second. Velocities are measured in metre per second (in symbols, m/s).