Why the whole world uses the same units — and how to move between them.
Schau dir das zuerst an — Lehrerschmidt erklärt Maßeinheiten für Gewicht, Strecke, Zeit und Fläche.
Lehrerschmidt — "Maßeinheiten – Gewicht, Strecke, Zeit und Fläche" (~7 min)
Mia is visiting the United States for the first time. She notices something strange. Every sign, every recipe, every weather app uses different units from everywhere else she has lived:
| What she sees | In the US | Everywhere else |
|---|---|---|
| Road distance | miles (mi) | kilometres (km) |
| Body weight | pounds (lb) | kilograms (kg) |
| Temperature | Fahrenheit (°F) | Celsius (°C) |
| Drink volume | fluid ounces (fl oz) | millilitres (ml) |
| Height | feet and inches | metres and centimetres |
Question: Why does almost the entire world use one system, but the US uses another? And what makes the metric system easier to work with?
Before reading further: why do you think the metric system might be easier for science? Write one sentence. Think about how you move between kilometres and metres vs. miles and feet.
The SI system (Système International d'Unités) has seven base units. Everything else is built from these:
| Base unit | Symbol | Measures |
|---|---|---|
| metre | m | length |
| kilogram | kg | mass |
| second | s | time |
| kelvin | K | temperature |
| ampere | A | electric current |
| mole | mol | amount of substance |
| candela | cd | luminous intensity |
The metric system is built on powers of 10. Each prefix multiplies the base unit by a factor of 10:
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| Giga | G | × 1 000 000 000 (10⁹) | 1 GB = 10⁹ bytes |
| Mega | M | × 1 000 000 (10⁶) | 1 MW = 10⁶ watts |
| kilo | k | × 1 000 (10³) | 1 km = 1 000 m |
| — (base) | — | × 1 | 1 m, 1 g, 1 s |
| centi | c | × 0.01 (10⁻²) | 1 cm = 0.01 m |
| milli | m | × 0.001 (10⁻³) | 1 mm = 0.001 m |
| micro | μ | × 0.000 001 (10⁻⁶) | 1 μm = 10⁻⁶ m |
The key trick: moving between metric units is always ×10, ×100, or ×1000 — just shift the decimal point. No memorising that 1 mile = 1 760 yards = 5 280 feet.
You need: a ruler or tape measure and a piece of paper.
Mia runs 2.4 km during training. How many metres is that? How many centimetres?
A horse needs 35 ml of a vitamin supplement per kilogram of body weight. Bella weighs 480 kg. How many litres of supplement does she need? (1 litre = 1 000 ml)
Convert each measurement. Show your working.
| Given | Convert to | Answer |
|---|---|---|
| 3.5 km | m | ___ |
| 8 200 m | km | ___ |
| 0.75 kg | g | ___ |
| 4 500 ml | litres | ___ |
| 2.3 cm | mm | ___ |
| 650 cm | m | ___ |
Mia's family drives through the US. The speed limit sign says 65 mph (miles per hour). 1 mile ≈ 1.609 km. What is this speed in km/h? Round to the nearest whole number.
A recipe calls for 2 cups of flour. 1 cup = 236.6 ml. Mia only has a scale (in grams). Flour has a density of about 0.57 g per ml. How many grams of flour does she need? Round to the nearest gram.
Order these lengths from shortest to longest: 1.2 m, 85 cm, 1 400 mm, 0.9 m, 110 cm. Show how you converted them all to the same unit first.
A scientist measures a bacterium at 3.5 μm (micrometres). Write this length in: (a) millimetres, (b) metres. Use powers of 10 in your answer.
Mia's suitcase weighs 23 kg. The airline limit is 50 lb. 1 kg ≈ 2.205 lb. Is her suitcase within the limit? By how many kg is she under or over?
The Mars Climate Orbiter was lost in 1999 because one team used metric units and another used imperial. The spacecraft received thruster data in pound-force seconds but expected newton-seconds (1 lbf·s ≈ 4.448 N·s). If the thruster fired with 12.0 lbf·s of impulse, what was the actual impulse in N·s? And what value did the software incorrectly assume it was (treating the number 12.0 as already being in N·s)? What was the error factor?
Find three things around you right now and measure or look up their measurements. Record them in at least two different units each (e.g. your height in cm, m, and feet). Which unit feels most natural for each thing, and why?