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…
1.6 — Prefixes and scientific notation
Physics deals with scales that span from the sub-atomic to the astronomical. The mass of an electron is around $10^{-30} mathrm{kg}$; the mass of…
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…
Significant figures, precision, and accuracy
Every number in physics carries a hidden claim about how uncertain it is. When we write 4.53 m, we are implicitly saying that we’re confident of the…
Unit conversion — the factor-label method
In §1.7 we saw that any unit can be written as a fraction equal to one, and chain-multiplying those fractions cancels unwanted units. That was just…
