Length, mass, and time are the three fundamental dimensions on which the measurement of all physical quantities is dependent.

EO 1.1DEFINE the three fundamental dimensions: length, mass, and time.

EO 1.2LIST standard units of the fundamental dimensions for each of the following systems:

a. International System of Units (SI)

b. English System

EO 1.3DIFFERENTIATE between fundamental and derived measurements.

Fundamental Dimensions

Physics is a science based upon exact measurement of physical quantities that are dependent upon three fundamental dimensions. The three fundamental or primary dimensions are mass, length, and time. These three fundamental units must be understood in order to lay the foundation for the many concepts and principles presented in this material.

Mass

Mass is the amount of material present in an object. This measurement describes "how much" material makes up an object. Often, mass and weight are confused as being the same because the units used to describe them are similar. Weight (a derived unit, not a fundamental unit) is a measurement that describes the force of gravity on the "mass" of an object.

Length

Length is the distance between two points. The concept of length is needed to locate the position of a point in space and thereby describe the size of a physical object or system. When measuring a length of pipe, the ends of the pipe are the two points and the distance between the two points is the length. A typical unit used to describe length is the "foot."

Time

Time is the duration between two instants. The measurement of time is described in units of seconds, minutes, or hours.

Units

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.