What a measurement carries
You ask a shop for "2 of rice". Two what?
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- A number alone says little.
- A unit tells what the number counts.
- The same rice is 2 kg or 2000 g.
- Science shares symbols and units.
The idea
The unit answers that question. It could be 2 kg or 2 g.
Science uses words with care. Words like pressure, energy, cell and solution have precise meanings in science.
Science also shares symbols and units. A quantity is something you can measure. A symbol is a letter that stands for it. A unit is the agreed amount you measure with. SI units are the standard units used in science worldwide. Time and length each have a symbol and a defined unit.
| Quantity | Symbol | Unit |
|---|---|---|
| Time | t | second (s) |
| Length | l | metre (m) |
| Mass | m | kilogram (kg) |
| Electric current | I | ampere (A) |
The letter m is the symbol for mass. The letter m is also the unit metre. Take care which one is meant.
A kilogram means the same everywhere. Measurements use agreed international standards, not local objects or opinions. So results can be compared, and trade stays fair.
Mixing up units causes real errors. Reading 2 kg as 2 g shrinks the amount 1000 times. Using SI units everywhere avoids many conversions.