Coherent derived units
Coherence means no extra numerical factor in the base-unit product.
Key knowledge
A coherent derived unit, for a stated quantity system and selected base units, is expressed as a product of powers of those base units with numerical factor one. In the SI, m/s is coherent for speed while km/h is a compatible unit requiring a conversion factor. Coherence is a relationship to a unit system; it is not a claim that every permissible unit is coherent.
Conditions and scope
- The quantity system and selected base units must be stated.
- Changing base units or quantity equations changes the coherence question; SI prefixes do not in general preserve coherence relative to the unprefixed SI base units.
Sources · 1
References
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Evidence and review
Read VIM1.12 definition and notes1–4 on the dependence on a quantity system/base units and coherent-unit examples. The speed example is a direct base-unit/product application; exclude any blanket claim that prefixed or accepted non-SI units are coherent.
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