Greatness, value, unity

‘The table is 2’ doesn’t mean anything. Two what? Two metres, two feet, two tables?

In physics, a measurement result always consists of two parts:

2,3 m
 |   |
 |   +--- l'unite : ce a quoi on compare
 +------- la valeur : combien de fois

A number without a unit is not a result. This is the primary source of error in exam papers, and the main cause of space probe crashes — the Mars Climate Orbiter was lost in 1999 because two teams were working one in metric units, the other in imperial units.

Quantity, unit: do not confuse them

• a quantity is what is being measured: a length, a duration, a mass
• a unit is the standard against which it is compared: the metre, the second, the kilogram

The same quantity can be expressed in several units. Duration is measured in seconds, minutes or years — it is always a duration.

The seven base units

The entire International System (SI) is based on seven units. The first five are sufficient to get started:

Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Duration	second	s
Electric current	ampere	A
Temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

All other units are derived from these. The newton, the volt and the joule are simply combinations of these seven.

Fun fact: the base unit of mass is the kilogramme, not the gramme. It is the only base unit whose name already contains a prefix — a historical accident that the SI has never corrected.

The symbol is not an abbreviation

We write m, not m. or M. Symbols never take a full stop and are case-sensitive: m is a metre, M is the prefix ‘mega’. Writing 5 Mm instead of 5 mm changes the result by a factor of one thousand billion.