
Arturo O. answered 04/12/18
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I suspect that the coefficient of 0.23 is really some constant that has a value of 0.23 in SI units, instead of a pure number. If so, its units must be ms/kg so that when multiplied by kg/s2, you get m/s.
Then
vf = k(ρs - ρo)gr2
where
k = 0.23 ms/kg
It probably comes from the slope of a linear plot of measured terminal speed vs. measured (ρs - ρo)gr2. Does this sound reasonable?