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Defi nition
A unit describes the structure of a physical unit.
It is a stereotype ?«unit?» of the UML element instance specification.
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5 The international unit system SI (acronym of the French name Le Syst??me International d??™unit?©s)
is the most commonly used unit system for physical units. It defines many basic units, including
meter, kilogram, second, ampere, Kelvin, mole and candela.
4.5 Block Diagrams
FIGURE 4-36
SI units.
Defi nition
A dimension describes the quantity of a unit.
It is a stereotype ?«dimension?» of the UML element instance specification.
Many values are normally bound to units in systems engineering models to prevent
system errors due to a faulty interpretation of values without units. A tragic
real-world example for such errors was the failed Mars mission Mars Climate
Orbiter . The probe was lost on Mars due to a unit error in the navigation system.
While NASA computed using SI units 5 (Newton), a part of the system used the
imperial unit system (pound) [14].
A unit describes a specifi c physical unit.


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