Physical quantities and measurement accuracy

The parameters of a body or phenomenon that can be measured in an experiment are called physical quantities: length, mass, time, volume, temperature. To express a quantity we need its numerical value and its unit: in “a lesson lasts 45 minutes”, 45 is the numerical value and minute is the unit. Measuring means comparing the measured quantity with a standard. Because using different units in different countries was inconvenient, the International System of Units (SI) was adopted in 1960; it has 7 base units: length — metre (m), mass — kilogram (kg), time — second (s), electric current — ampere (A), temperature — kelvin (K), luminous intensity — candela (cd), amount of substance — mole (mol). When a quantity is very large or very small, prefixes are used: kilo (1000), mega (1 000 000), hecto (100), deca (10), deci (0.1), centi (0.01), milli (0.001), micro (0.000001). For example, 1 km = 1000 m and 1 mm = 0.001 m. Before comparing quantities, express them in the same unit; quantities of different kinds, such as length and time, cannot be compared.