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Everyone agreeing

Why Do We Measure With Units?

A measurement is a count of agreed-size steps. Without agreement it means nothing.

Measured in feet — whosefeet??Different answers everytimeMeasured in centimetres The same answer foreveryone
A unit is a promise about size.

  1. Step 1 of 5

    Measuring is counting equal steps.

    1 Pick a unit2 Lay it end toend33Count how many

    Lay a stick end to end along a table and count. Seven sticks long. That is a measurement — a count of a repeated unit.

  2. Step 2 of 5

    The unit has to stay the same size.

    Uneven steps The count means nothingEqual steps The count istrustworthy

    If your steps vary, your count is meaningless. Overlapping or leaving gaps ruins it too — the units must sit end to end exactly.

  3. Step 3 of 5

    And everybody has to agree on it.

    A king's foot Changes with each kingA defined metre The same everywhere,forever

    Old units were body parts — a foot, a cubit, a hand. They varied from person to person, which made trade a constant argument.

  4. Step 4 of 5

    Modern units are pinned to physics.

    1 Light's speedis fixed2 Time is definedby atoms3 So the metrenever changes

    A metre is now defined as how far light travels in 1/299,792,458 of a second. Nothing about it can drift or wear out.

  5. Step 5 of 5

    Pick a unit that fits the job.

    Journey — kilometres12 kmRoom — metres4 mScrew — millimetres6 mm

    Millimetres for a screw, metres for a room, kilometres for a journey. The right unit gives a number you can actually hold in your head.

    A Mars orbiter was lost in 1999 because one team used metric units and another used imperial.

The short version

Measuring is counting agreed, equal-sized units — the agreement is what makes the number useful.

Try it yourself

Measure your table in hand-widths, then have someone else do it. Compare the two numbers and see the problem.

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