System of Units
Simple Explanation
A system of units is a complete, agreed-upon set of units for every physical quantity. The International System of Units (SI) — built on the metre, kilogram and second — is the system used almost everywhere in science today. Older systems include the CGS system (centimetre-gram-second) and the FPS system (foot-pound-second).
Why Do We Need It?
Using one shared system worldwide means a measurement made in one country means exactly the same thing in every other country — essential for science, trade, medicine, and engineering to all work together safely.
Worked Example
Convert a CGS measurement into SI units
A length is measured as 250 cm in the CGS system. Express it in the SI unit of length (metres).
Why Does This Work?
Every system of units is internally consistent, so converting between systems is always just a matter of multiplying or dividing by the fixed ratio that relates the two units for the same quantity.
Real-Life Example
A 1999 spacecraft mix-up
NASA's Mars Climate Orbiter was lost in 1999 partly because one engineering team used pound-force units (an old imperial system) while another used SI newtons for the same calculation.
The unnoticed mismatch between the two systems threw off a critical navigation calculation, showing how essential it is for everyone working together to use the same system of units.
Practice
Which system of units is now used almost universally in science?
EasyA mass is measured as 3.2 kg in SI units. Express this mass in grams (the CGS unit of mass).
MediumCommon mistake
Mixing units from two different systems within the same calculation (like using centimetres in a formula that expects metres) without converting first — this is one of the most common sources of wrong answers in physics.
Quick Review
- SI (metre, kilogram, second) is the internationally agreed system of units used in modern science.
- CGS (centimetre-gram-second) and FPS (foot-pound-second) are older systems, still occasionally seen.
- Always convert every quantity into one consistent system before calculating.