Physical quantities and SI units কী?

Example 1.4 (Reading prefixes)

Green light has wavelength about $500\,\mathrm{nm} = 5.00 \times 10^{-7}\,\mathrm{m}$; a human hair is about $70\,\text{µ}\mathrm{m} = 7.0 \times 10^{-5}\,\mathrm{m}$ thick; the Moon orbits at $384\,\mathrm{Mm} = 3.84 \times 10^{8}\,\mathrm{m}$. One raindrop has mass about $30\,\mathrm{mg} = 3.0 \times 10^{-5}\,\mathrm{kg}$ — 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 $d = 1.27 \times 10^{7}\,\mathrm{m}$ (three significant figures), so its circumference is $\pi d = 3.141\,59 \times 1.27 \times 10^{7}\,\mathrm{m} = 3.99 \times 10^{7}\,\mathrm{m}$ — three figures, not the eight the calculator displays. Writing $39\,898\,227\,\mathrm{m}$ would claim to know the planet to the metre.