Why we need units
A measurement needs a number and a unit. "5" means nothing, but "5 metres" does. The metric system uses units that change by powers of ten, so converting is easy.
| Quantity | Units (small to large) | Links |
|---|---|---|
| Length | mm, cm, m, km | 10 mm = 1 cm; 100 cm = 1 m; 1 000 m = 1 km |
| Mass | mg, g, kg, tonne | 1 000 g = 1 kg; 1 000 kg = 1 tonne |
| Capacity | ml, litre | 1 000 ml = 1 litre |
| Time | second, minute, hour, day | 60 s = 1 min; 60 min = 1 hour; 24 hours = 1 day |
Converting step by step
Try itKilometres, metres, centimetres
Slide the distance and see all three units. The graph shows that metres grow in a straight line with kilometres.
Worked exampleA mixed problem
A bag has a mass of 3 500 g. Another bag has a mass of 2.2 kg. What is their total mass in kg?
- Change grams to kilograms: divide by 1 000.
- Add.
Answer: The total mass is 5.7 kg.
Key terms
- Unit
- A standard amount used to measure something.
- Metric system
- Units based on powers of ten, such as metres and kilograms.
- Conversion
- Changing a measurement from one unit to another.
Summary
- Always give a unit with a measurement.
- Smaller unit: multiply. Larger unit: divide.
- Know the links: 1 km = 1 000 m, 1 m = 100 cm, 1 kg = 1 000 g, 1 litre = 1 000 ml.
- Time is not metric: 60 seconds in a minute, 60 minutes in an hour.