Whose Hand?
Why the world needed standard units
What this lesson covers
Why it matters
Long ago, cloth was sold by the arm-length — the shopkeeper's arm. Shoppers with short-armed shopkeepers paid more for less. The fix changed the world: one metre, everywhere, for everyone.
Predict first
Three people measure a rope in hand-spans and get 38, 45 and 33. Who is wrong?
Each hand is a different 'unit'. Measurement needs a unit that is the SAME for everyone — that's why the metre exists.
- nobody — their hands are different sizes; the UNIT was the problem — correct
- all of them — bad counting
- the one with 45
What you do
Measure the rope with three different hands and then the metre rule — feel the chaos, then the fix.
Check yourself
Length, mass and time are measured in (S.I.)…
The big three of measurement — every lab on Earth agrees on them.
A good unit must be…
Fixed and universal — or the cloth-shop cheating returns.
To measure your pencil you would use…
Pick the tool that fits the job — centimetres for pencils, metres for rooms, kilometres for cities.
- metre, kilogram, second — correct
- hand, stone, heartbeat
- litre, degree, watt
- the same for everyone, everywhere, always — correct
- as big as possible
- different in each shop
- a 15 cm ruler — correct
- a measuring tape of 50 m
- a weighing scale