Slide 1
A unit tells us what the measured number counts
A measurement such as five metres combines a number with a unit. The unit tells another person what the number counts.
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The same length can be written as 5 metres or 500 centimetres. Nothing physical has changed: the numerical value compensates for the smaller unit. Losing the unit can therefore create a hundredfold misunderstanding.
Slide 2
Speed combines distance and time units
Speed combines distance and time. Its unit, metres per second, describes how much distance is covered during each second.
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Area needs two length factors, so one square metre contains ten thousand square centimetres. Squaring the conversion factor matters; multiplying by one hundred only converts a length.
Slide 3
SI gives measurements a shared system
The International System of Units (SI) provides agreed base units. Their combinations let laboratories compare quantities across subjects and national borders.
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A newton, the unit of force, equals a kilogram metre per second squared. A special unit name can hide a combination of simpler units, so equations remain connected to the quantities they describe.
Slide 4
Prefixes change scale while keeping the quantity
A prefix changes the size of a unit by an agreed factor. Millimetres and kilometres remain measurements of length.
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Milli means one thousandth and kilo means one thousand. Prefixes keep very small or large values readable, but symbols are case sensitive: a misplaced capital can change a stated quantity enormously.
Slide 5
An exact definition does not remove measurement uncertainty
Modern SI definitions use fixed physical constants. Real instruments still need calibration and still produce results with measurement uncertainty.
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Defining the metre through light does not make every ruler exact. A laboratory must realize the unit through a practical procedure, then transfer that reference to other instruments through comparisons.
Sources
- SI Units
National Institute of Standards and Technology
- Definitions of SI Base Units
National Institute of Standards and Technology