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Physics · Glossary

What is Physical quantities and SI units?

Also known as: physical quantity · unit · International System of Units (SI) · metre · kilogram · second

Definition 1.1 High School Physics · Chapter 1 — Orders of Magnitude: Measuring the Universe

A physical quantity is a property of the world that can be measured: a length, a duration, a mass. Measuring it means counting how many times a reference quantity — the unit — fits into it. The International System of Units (SI) fixes seven base units, of which three run this whole volume: the metre (m\mathrm{m}) for length, the kilogram (kg\mathrm{kg}) for mass and the second (s\mathrm{s}) for time. (The other four — ampere, kelvin, mole, candela — will join when electricity, heat and chemistry enter the story.)

Examples

Example 1.4 (Reading prefixes)

Green light has wavelength about 500 nm=5.00×10−7 m500\,\mathrm{nm} = 5.00 \times 10^{-7}\,\mathrm{m}; a human hair is about 70 µm=7.0×10−5 m70\,\text{µ}\mathrm{m} = 7.0 \times 10^{-5}\,\mathrm{m} thick; the Moon orbits at 384 Mm=3.84×108 m384\,\mathrm{Mm} = 3.84 \times 10^{8}\,\mathrm{m}. One raindrop has mass about 30 mg=3.0×10−5 kg30\,\mathrm{mg} = 3.0 \times 10^{-5}\,\mathrm{kg} — note that the SI base unit of mass, alone among the seven, carries a prefix already.

Example 1.10 (Earth’s circumference)

Earth’s diameter is d=1.27×107 md = 1.27 \times 10^{7}\,\mathrm{m} (three significant figures), so its circumference is πd=3.141 59×1.27×107 m=3.99×107 m\pi d = 3.141\,59 \times 1.27 \times 10^{7}\,\mathrm{m} = 3.99 \times 10^{7}\,\mathrm{m} — three figures, not the eight the calculator displays. Writing 39 898 227 m39\,898\,227\,\mathrm{m} would claim to know the planet to the metre.

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