SI units

SI units are the worldwide standard set of units that scientists and most countries agree to use: metres for length, kilograms for mass, seconds for time, and so on. SI stands for the French Systeme International d'Unites, the International System of Units. The idea is simple: if everyone uses the same rulers, our numbers can be compared and combined without confusion.

The system is built from seven base units, and every other unit is derived from them by multiplication and division (for example speed in metres per second, m/s). SI also comes with a tidy set of prefixes for powers of ten, such as kilo- (times 1000), centi- (times 1/100), milli- (times 1/1000), mega- (times 1,000,000), so one kilometre is 1000 metres and one millisecond is 0.001 second. Because everything scales by tens, converting within SI is just moving a decimal point.

Using SI consistently is what lets a formula work: if you feed every quantity in its SI unit, the answer comes out in the correct SI unit automatically. Mixing systems (say, pounds with metres) is a classic source of real engineering disasters, which is why physics courses insist on SI.