Family of Units and their Conversions



Family of Units and their Conversions

Introduction:

There are three, soon to be only two, family of units: SI, cgs, and FPS. The first two are widely used throughout physics and the third is archaic and only used in America. It is slowly being replaced by the more scientific SI and cgs.

What is a “family” of units? All calculations are derived from only a few fundamental quantities. The fundamental quantities are those quantities accepted as the basis for all of the other units in that family. These other units are therefore called derived.

The three families of units:

English System: (never used in lab, but ever present in everyday life)

Units: FPS: designates foot, pound, and second is an old system developed by the English back in the 15th century.

Metric System: all of the units are related by powers of 10.

cgs: This designates centimeter, gram, and second developed by an international scientific committee based on the SI system.

SI: This designates meter, kilogram, and second developed by an international scientific committee.

The logic of the systems uses three very fundamental physical quantities: length, mass, and time. These quantities are pervasive throughout the physical universe – both classical and quantum. As such it is essential that these be chosen as fundamental units upon which the remaining derived units are defined.

NOTE: one might ask at this point, isn’t “pound” a unit of force, and not a unit of mass? The answer to the question is “yes”. And so you see the underlying problem with this old system of units. Pounds are a force and so they depend on gravity. Mass however is free of such a dependence. (FPS will no longer be mentioned in class.)

The following is a table of the SI units presented throughout Physics I.

|distance |m | | |

|mass |kg | | |

|time |s | | |

|speed |m/s | | |

|momentum |kg.m/s | | |

|acceleration |m/s2 | | |

|force |kg.m/s2 |N (Newton) | |

|energy |kg.m2/s2 |N.m |J (Joule) |

|power |kg.m2/s3 |J/s |W (Watt) |

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