Chapter 1 — Measurement

Learn why physics without measurement is meaningless — SI units, length, time and mass standards, dimensional analysis, significant figures, and unit conversion.

1.1 — Why we measure

Physics is built on measurement. You can describe nature with elegant words, but until you can pin a property to a number and compare with what a…

1.2 — The SI system

Almost all of modern physics uses the International System of Units (SI). It defines just 7 base units, and every other quantity is derived from these.

1.3 — The meter: length standard

The meter has been redefined several times, each redefinition driven by the same motivation: anchor the unit to something more stable and more…

1.4 — The second: time standard

Of the seven SI base units, the second is by far the most precisely realized. Modern atomic clocks reach accuracies orders of magnitude beyond any…

1.5 — The kilogram: mass standard

The kilogram was the last SI base unit tied to a physical object — a platinum-iridium cylinder locked in a vault outside Paris — until 2019. The…

Dimensional analysis

Behind every physics equation lies a hidden claim of the same rank: both sides describe the same kind of quantity. If one side has the dimensions of…