1.1 Units & 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 in between.
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
[Image of table of 7 SI base 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.
| 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 |
Derived Units
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
Unit = 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 burn up in the atmosphere.