Summary

Physics is an experimental science, and every measurement must be expressed in units -- a bare number, without a unit, says nothing physical. Every physical quantity has both a magnitude and a unit.

• The SI units of length, mass, time, temperature, electric current, amount of substance and luminous intensity are the metre, kilogram, second, kelvin, ampere, mole and candela respectively; every mechanical, electrical, magnetic and thermal unit is derived from these seven.
• Small distances are measured with a screw gauge or vernier caliper; large distances with parallax or RADAR methods.
• The uncertainty in a measurement is its error. Accuracy measures closeness to the true value; precision measures the reproducibility of repeated readings. Every accurate measurement is precise, but a precise measurement need not be accurate.
• Adding or subtracting quantities: the result is only as precise (as many decimal places) as the least precise term. Multiplying or dividing quantities: the result keeps only as many significant figures as the factor with the fewest. …