Physical quantities and units
A physical quantity is a number together with a unit; SI fixes seven base units that every other unit is built from.
Practise this conceptA physical quantity is always a number paired with a unit — "5" on its own means nothing in physics, but "5 metres" describes something real. Dropping the unit doesn't just lose detail, it loses the entire meaning.
The SI system fixes seven base units — the metre, kilogram, second, ampere, kelvin, mole and candela — and builds every other unit from combinations of those seven. A newton, for instance, is just a shorthand for kilogram times metre per second squared; writing out force in base units instead of newtons would work exactly the same, only more clumsily.
Keeping units attached through a calculation is also a built-in error checker: if a formula for a speed spits out an answer in kilograms, something has gone wrong in the algebra, unit mismatches are one of the fastest ways to catch a mistake before it becomes a wrong final answer.
Key ideas
Unlocks: Atoms and the nucleus, Electric charge and current, Measurement, precision and accuracy, Reflection and refraction, Scalars and vectors, Speed and motion, Temperature and heat, Waves and oscillations
Common misconceptions
- Units are just labels that can be added to any numeric answer at the end, not part of the calculation itself.
- A number without a unit is still a complete physical quantity.
- All SI base units can be derived from length and time alone.