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| - * [Wikipedia on "Physical Units"|http://en.wikipedia.org/wiki/Physical_unit]. Some interesting sentences: "Units as Dimensions: Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor: Q = n * Q |
| + * [Wikipedia on "Physical Units"|http://en.wikipedia.org/wiki/Physical_unit]. Some interesting sentences: "Units as Dimensions: Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor: Q = n * [Q] = n[Q]. The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. In formulas the unit [Q] can be treated as if it was a kind of physical dimension: see dimensional analysis for more on this treatment. A distinction should be made between units and standards. A unit is fixed by its definition, and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions. For example, the metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be one metre long only at a certain temperature. For most quantities a unit is absolutely necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given." |
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| + * [Wikipedia on "Dimensionless Numbers"|http://en.wikipedia.org/wiki/Dimensionless_number]. "In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters one's system of units of measurement, for example from English units to metric units. Such a number is typically defined as a product or ratio of quantities which do have units, in such a way that all units cancel. There are infinitely many dimensionless numbers. Some of those that are used most often have been given names, as in the following list of examples (in alphabetical order, indicating their field of use): Dispersion, Archimedes number, Biot number, Bodenstein number, Capillary number, ..." |
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| - * Physical Quantity. |
| - * Metric. |
| - * |
| + * Physical Quantity |
| + * Metric |
| + * Standard |
| + * Unit |
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