Time is the duration between two instants. The measurement of time is described in
units of seconds, minutes, or hours.
A number alone is not sufficient to describe a physical quantity. For example, to say that "a
pipe must be 4 long to fit" has no meaning unless a unit of measurement for length is also
specified. By adding units to the number, it becomes clear, "a pipe must be 4 feet long to fit."
The unit defines the magnitude of a measurement. If we have a measurement of length, the
unit used to describe the length could be a foot or yard, each of which describes a different
magnitude of length. The importance of specifying the units of a measurement for a number
used to describe a physical quantity is doubly emphasized when it is noted that the same
physical quantity may be measured using a variety of different units. For example, length may
be measured in meters, inches, miles, furlongs, fathoms, kilometers, or a variety of other units.
Units of measurement have been established for use with each of the fundamental dimensions
mentioned previously. The following section describes the unit systems in use today and
provides examples of units that are used in each system.
There are two unit systems in use at the present time, English units and International System
of Units (SI).
In the United States, the English system is currently used. This system consists of various units
for each of the fundamental dimensions or measurements. These units are shown in Table 1.