Why Units Rule

Why it matters

Three people measure the same rope with their hands: 38, 45 and 33 'hands'. All careful, all honest, all useless — until someone brings a metre rule.

The idea in plain words

The grammar of every measurement. Tap the terms.

physical quantity = number × unit

Which is correct: '10 Kgs', '10 kgs' or '10 kg'?

10 kg

• the atoms of measurement: fundamental units — the S.I. base seven — metre (length), kilogram (mass), second (time) lead the list
• the molecules: derived units — built by combining bases: m/s, kg·m/s², joule…
• writing rules: unit rules — symbols never pluralised (5 kg, not kgs), full names lowercase (newton), symbol capital only if from a name (N)
• Symbols take no plural and no capital K
• kg is the S.I. symbol for kilogram

Predict first

Why did the three hand-span measurements disagree?

Each hand IS a different unit. Measurement = number × unit, so science demands units that are fixed, universal and convenient — the S.I.

• Their unit (a hand) was different for each person — correct
• Someone counted wrongly
• The rope kept changing length

What you do

Measure the rope with all three hands, then with the metre rule — feel why the world signed the S.I. treaty.

Check yourself

A good standard unit must be…

The whole point is that YOUR metre equals MY metre — everywhere, always.

The S.I. unit of mass is the…

Oddly, the BASE unit carries the 'kilo' — the kilogram, not the gram.

Which is a DERIVED unit?

The newton is assembled from three base units — force is a derived quantity.

• invariable, universally accepted and reproducible — correct
• as large as possible
• different in each country
• kilogram — correct
• gram
• pound
• newton (kg·m/s²) — correct
• second
• metre