Chapter 2 · Measurement and Problem Solving
2.5The Basic Units of Measurement
10 min · two checks
Predict
What is the SI base unit of mass, and why is “kilogram” an odd member of that list?
The idea
- List the SI base units a first course actually uses.
- Apply prefix multipliers and name two derived units.
SI builds measurements from base units. Length is the meter (m), mass the kilogram (kg), time the second (s), temperature the kelvin (K), and amount of substance the mole (mol). Electric current, the ampere, shows up later. Prefixes scale those units by powers of ten: milli- is 10⁻³, centi- is 10⁻², kilo- is 10³, micro- is 10⁻⁶, nano- is 10⁻⁹. 2.5 km is 2.5 × 10³ m. Because the kilogram already includes kilo-, a thousand kilograms is a megagram, not a “kilokilogram.”
Derived units are combinations. Volume in the lab is usually the liter, and 1 mL = 1 cm³ exactly. Energy is the joule, 1 J = 1 kg·m²/s². Density is mass over volume. Pressure is force over area; the pascal is 1 N/m². You will meet kPa constantly in gas problems. Prefix skill is unit conversion in disguise.
Keep these
- Base units to memorize now: m, kg, s, K, mol.
- milli 10⁻³, centi 10⁻², kilo 10³, micro 10⁻⁶.
- 1 mL = 1 cm³. The joule and the pascal are derived units.