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.

Quantity	SI base unit	Symbol
Length	meter	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Derived units are combinations of these. For example:

• Velocity: m/s
• Acceleration: m/s^2
• Force: kg·m/s^2, given the shorthand name newton (N) so equations stay readable
• Energy: kg·m^2/s^2, named joule (J)
• Pressure: kg/(m·s^2) = N/m^2, named pascal (Pa)

Why only 7 base units? Because every other quantity in mechanics, electromagnetism, thermodynamics, and chemistry can be built from them. There is nothing magical about the number 7 — it is a minimal sufficient set, chosen by international agreement.

Pet peeve. Students sometimes write kgms-2. Don't. Write kg·m/s^2 or kg·m·s^{-2}. Dots and spaces matter for readability — and graders notice.

What you should be able to do

After this section you should be able to:

• Recall the seven SI base units and their symbols from memory
• Decompose any derived unit (N, J, Pa, W, ...) back into base units
• Explain why km/s^2 equals 1000 m/s^2, not 1000000 m/s^2

⇧ Back to chapter