Unit Annotations and Context - Maple Help
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Unit Annotations and Context

 

Description

Examples

Description

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In Maple, units (and dimensions) can be annotated with an arbitrary symbol to indicate that the physical quantity being described is of a particular type. This is done by writing the unit as a function of the symbol, as . The symbol is called an annotation.

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A few commands related to the Units package have an option called symbolic; this option makes Maple temporarily ignore these annotations.

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Additionally, a unit can be indexed with specific symbols: this indicates an alternative definition of the unit itself, not just a different underlying physical quantity. The index is called the context. If no context is specified for a unit, the default context is used. Which context is the default context differs per unit; the help pages with tables of units, such as Units,time or Units,permeability, indicate default contexts with an asterisk (*).

Examples

One can specify a volume as  to indicate  gallons of gasoline or diesel as opposed to  gallons in the abstract, or  to indicate  joules of mechanical energy rather than general energy. The effect of this is that the annotated units do not recombine with un-annotated units. This is nice for discussing fuel consumption, for example: without this feature, the unit mile_per_gallon would simplify to approximately , since its dimension is ; however,  does not simplify (its dimension is ).

One obtains an angle in radians by dividing the length of an arc by the length of a radius. At first sight, this would seem to require that an angle always simplifies to unit 1, which would be undesirable. The solution is that in Maple, the denominator gets a unit of the dimension ; for example, . So a radian is defined as a .

As an example of contexts, Maple understands a mile[standard] as different from a mile[US_survey]: one standard mile is  US survey miles. Or for a different example: minute can refer to a unit of time, but also to  of a degree - the unit of angle. In Maple, these are known as the minute[SI] (or just minute, since SI is the default context for minute, as can be seen on the help page Units,time) and the minute[angle], respectively.

If both a context and an annotation are needed on a unit, then the context should be specified first. For example, a circle segment with a radius of  and an arc length of  describes an angle of .


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