Physics » The Nature of Physics » Physical Quantities and Units

Summary and Main Ideas

Summary of lessons so far

The following are the main ideas from the past few lessons:

  • Physical quantities are a characteristic or property of an object that we can measure or calculate from other measurements.
  • Units are standards for expressing and comparing the measurement of physical quantities. We can express all units as combinations of four fundamental units.
  • The four fundamental units we will use in this text are the meter (for length), the kilogram (for mass), the second (for time), and the ampere (for electric current). These units are part of the metric system, which uses powers of 10 to relate quantities over the vast ranges encountered in nature.
  • Scientists abbreviate the four fundamental units as follows: meter, m; kilogram, kg; second, s; and ampere, A. The metric system also uses a standard set of prefixes to denote each order of magnitude greater than or lesser than the fundamental unit itself.
  • Unit conversions involve changing a value expressed in one type of unit to another type of unit. This is done by using conversion factors, which are ratios relating equal quantities of different units.

Glossary of words

Physical quantity

a characteristic or property of an object that can be measured or calculated from other measurements


a standard used for expressing and comparing measurements

SI units

the international system of units that scientists in most countries have agreed to use; includes units such as meters, liters, and grams

English units

system of measurement used in the United States; includes units of measurement such as feet, gallons, and pounds

Fundamental units

units that can only be expressed relative to the procedure used to measure them

Derived units

units that can be calculated using algebraic combinations of the fundamental units


the SI unit for time, abbreviated (s)


the SI unit for length, abbreviated (m)


the SI unit for mass, abbreviated (kg)

Metric system

a system in which values can be calculated in factors of 10

Order of magnitude

refers to the size of a quantity as it relates to a power of 10

Conversion factor

a ratio expressing how many of one unit are equal to another unit

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