1.4:
Units and Standards of Measurement
A physical quantity is any number that describes a physical phenomenon quantitatively. For instance, length is a physical quantity that describes the distance between two points.
The measurements of physical quantities are expressed in terms of units. The mass of your body can be expressed in kilograms and your height in terms of meters.
Physical quantities expressed without standard units are difficult to understand. For example, if the height of the Eiffel Tower is reported to be 300, without providing units the context of the number is unclear. This means the Eiffel tower could be relatively tall, or very short.
In order to measure and report the various properties of matter accurately, a common system of measurement has been developed known as the International System of Units or the SI units.
The SI system recommends using seven base quantities and their units, which form the International System of Quantities.
1.4:
Units and Standards of Measurement
A physical quantity is defined either by specifying its measurement method or by stating how it is calculated from other measurements. For example, consider a metallic cube. We might define its mass and dimensions by specifying methods for measuring them, such as using a weighing machine and a meter scale. Then, we could define the volume by stating that it is the cube of its side, and we could calculate the density as the mass divided by the volume.
Measurements of physical quantities are expressed in terms of units, which are standardized values. For instance, the length of the side of a cube, which is a physical quantity, can be expressed in units of meters or centimeters, depending upon the size of the cube.
There are mainly two systems of units in use. The first is the metric system, which uses SI units, and the second is the customary or imperial system, which uses English units. SI is the international abbreviation for the French Système International d'Unités. It is the most widely used unit system globally and is considered the standard system by scientists and mathematicians.
This text is adapted from Openstax, University Physics Volume 1, Section 1.2: Units and Standards.