1.1 Basics - Units and Measurements
A unit is a standard amount used to measure a physical quantity. We need units for almost everything, from measuring a dose of medicine to building a rocket, and a million things inbetween.
There are hundreds of units. For example, we can measure mass in kilograms (kg), grams (g), or pounds (lb). It's very important that everyone uses the same units to avoid confusion or even deadly mistakes.
SI Units
In science, we use the SI system (Système International). This is a standard set of units that is used by scientists all over the world.
The SI system is built on seven base units for fundamental quantities like length, mass, and time.
| Quantity | Unit Name | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric Current | ampere | A |
| Temperature | kelvin | K |
| Amount of Substance | mole | mol |
| Luminous Intensity | candela | cd |
All other units can be built from these seven. These are called derived units. For example, speed is a derived quantity. The word formula is:
speed = distance / time
This gives the SI derived unit for speed as metres per second (m/s).
Why do units matter? In 1999, NASA's Mars Climate Orbiter was lost when it crashed into Mars. The reason was a simple mistake: one team of engineers used SI units (newtons) while another team used imperial units (pounds of force). This mistake caused the orbiter to get too close to mars, causing it to burn up and crash into the planet.