Physics » The Nature of Physics » Physical Quantities and Units

Physical Quantities and Units

By the end of this lesson and the next few, you should be able to:

  • Perform unit conversions both in the SI and English units.
  • Explain the most common prefixes in the SI units and be able to write them in scientific notation.

What is a physical quantity?

The range of objects and phenomena studied in physics is immense. From the incredibly short lifetime of a nucleus to the age of the Earth, from the tiny sizes of sub-nuclear particles to the vast distance to the edges of the known universe, from the force exerted by a jumping flea to the force between Earth and the Sun, there are enough factors of 10 to challenge the imagination of even the most experienced scientist.

earth-to-moon

The distance from Earth to the Moon may seem immense. But it is just a tiny fraction of the distances from Earth to other celestial bodies. Image Attribution: NASA

Giving numerical values for physical quantities and equations for physical principles allows us to understand nature much more deeply than does qualitative description alone. To comprehend these vast ranges, we must also have accepted units in which to express them. And we shall find that (even in the potentially mundane discussion of meters, kilograms, and seconds) a profound simplicity of nature appears—all physical quantities can be expressed as combinations of only four fundamental physical quantities: length, mass, time, and electric current.

We define a physical quantity either by specifying how it is measured or by stating how it is calculated from other measurements. For example, we define distance and time by specifying methods for measuring them. On the other hand, we define average speed by stating that it is calculated as distance traveled divided by time of travel.

What is a unit?

Measurements of physical quantities are expressed in terms of units, which are standardized values. For example, the length of a race is a physical quantity. We can express it in units of meters (for sprinters) or kilometers (for distance runners). Without standardized units, it would be extremely difficult for scientists to express and compare measured values in a meaningful way. (See image below.)

meaningless-distance

Distances given in unknown units are maddeningly useless. Image Attribution: OpenStax College Physics

Systems of Units

There are two major systems of units used in the world: SI units (also known as the metric system) and English units (also known as the customary or imperial system). English units were historically used in nations once ruled by the British Empire and are still widely used in the United States. Virtually every other country in the world now uses SI units as the standard. The metric system is also the standard system agreed upon by scientists and mathematicians. The acronym “SI” is derived from the French Système International.

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