What is Measuring a length? — Physics

Definition 7.3 Primary & Middle School Physics · Chapter 7 — Measuring Length

To measure a length is to count how many times a chosen unit — a step, a hand span, a small stick — fits along it, end to end with no gaps and no overlaps. The answer is a number of units: “the room is $12$ steps long.”

The same wall, paced twice: 12 small feet, or 8 big feet. Different feet, different numbers — the wall did not change. — The same wall, paced twice: $12$ small feet, or $8$ big feet. Different feet, different numbers — the wall did not change.

Examples

Example 7.4 (Try it: pace the room)

Cross your bedroom heel-to-toe, counting your foot-lengths. Ask a grown-up to do the same. You might count $18$ feet and the grown-up only $12$ — for the same room! Nobody counted wrong: your feet are shorter, so more of them fit. A measurement made with my foot cannot be checked with your foot.

Example 7.8 (Which unit for which job?)

Small things like a beetle, a stamp or your hand are measured in centimetres. Big things like a corridor, a bus or a swimming pool are measured in metres: the pool is $25\,\mathrm{m}$ long — imagine counting that in fingernail-widths! Choosing a sensible unit is half the work of measuring.

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