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@Arcaik @j_bertolotti @mansr @johncarlosbaez
That's kind of the point; the current variant of metric system, MKS, and the units people use, like litres, are not coherent (other metric variants include CMS and MTS). And while gramme is a unit of weight it is not a "base unit" so can't be used in other calculations, such as force = mass * acceleration.

"A coherent derived unit, for a chosen set of base units, is a product of powers of base units, with the proportionality factor being one."