# Fundamental Units of Measurement

by Ron Kurtus (revised 18 June 2013)

You measure things by defining a standard unit and then stating the measurement in terms of multiples of that unit. A * fundamental unit of measurement* is a defined unit that cannot be described as a function of other units.

The* International
System of Units (SI)* defines seven fundamental units of measurement. They can be applied to the various Physical Science areas of study. However, there are some questions about the definitions.

Distance, time and mass are the fundamental units.

- What is a unit of measurement?
- What are the seven fundamental units?
- what are some problem areas with these definitions?

This lesson will answer those questions. Useful tool: Units Conversion

## Measuring in units

In order to measure something, you need to define a *unit of measurement*. "Unit" refers to 1. In this way, all measurements are multiples of that unit. For example, the unit of mass is the kilogram. Thus, the measurement of mass is in multiples—or fractions—of 1 kilogram.

Originally, the English unit

footwas the length of the King's foot. Thus, a distance of 25 feet was 25 time the foot unit of measurement.Unfortunately, each King had a different sized foot, so that brought about some confusion. Finally, they agreed on a standard length for a foot that would not vary.

The units of measurement are defined as standard and do not vary.

## SI base units

The International System of Units (SI) defined seven basic units of measure from which all other SI units are derived.

These SI base units or commonly called metric units are:

Measure | Unit | Symbol | Area of Science |
---|---|---|---|

Time | Second | s | All |

Length or distance | Meter or Metre | m | All |

Mass | Kilogram | kg | Physics |

Electric Current | Ampere | A | Physics |

Temperature | Kelvin | K | Physics |

Luminous Intensity | Candela | cd | Optics |

Amount of Substance | Mole | mol | Chemistry |

Although these SI base quantities are supposed to be a set of mutually independent dimensions, some may well be interdependent.

## Problem areas

There are some variations or problem areas in these fundamental units.

### Length

Although multiples or fractions of a meter are useful in most sciences, the unit is impractical in Astronomy. Instead, the fundamental unit of length in Astronomy is the light-year, which is the distance light travels in kilometers in one year.

### Mass

It would seem more intuitive to define the fundamental of mass as a gram. However, the SI decision was to say that 1000 grams or a kilogram was fundamental.

### Electric current

Since electric current is man-made and depends on a number of factors, the ampere does not seem appropriate as a fundamental unit. The ampere is defined as:

"The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10

^{−7}newton per meter of length."

That does not seem too fundamental.

## Summary

Units of measurement that are considered fundamental, such that they cannot be described as a function of other units. They often related specifically to the various sciences. However, there are some problems with the definitions.

Look to the source of things

## Resources

### Websites

**Units of measurement** - Wikipedia

**General Tables of Units of Measurement** - National Institute of Standards and Technology

**Units of Measurement and SI Base Units** -
Includes conversions

**Fundamental unit** - Wikipedia

**Base Units of the International System (SI)** - University of North Carolina at Chapel Hill

**SI base unit** - Wikipedia

### Books

## Questions and comments

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units_fundamental.htm**

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## Where are you now?

## Fundamental Units of Measurement