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How can I get FindRoot to work with units?

James Bowery
Posted 2 years ago
POSTED BY: James Bowery
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Domen V
Domen V
Posted 2 years ago

Hm, right. It really seems like it cannot deal with a multivariable problem ... Here is a simpler example that produces the same error message:

FindRoot[{x == Quantity[1, "Seconds"], 
  y == Quantity[2, "Meters"]}, {{x, Quantity[3, "Seconds"]}, {y, 
   Quantity[4, "Meters"]}}]
(* FindRoot::nlu: -- Message text not found -- 
  ({3.s,4.m}) ({x,y}) ({2.s,2.m}) ({2}) (Removed[$$Failure]) *)
POSTED BY: Domen V
James Bowery
James Bowery
Posted 2 years ago

Thanks for finding a minimal example.
POSTED BY: James Bowery
James Bowery
James Bowery
Posted 2 years ago

Then perhaps I've found a bug in Mathematica's handling of Units with FindRoot because here's the opaque error message ("Message text not found") that appears when I don't strip off the Units with QuantityMagnitude:

enter image description here
POSTED BY: James Bowery
Domen V
Domen V
Posted 2 years ago

FindRoot does work with Quantity, but you have to provide the appropriate units for the variables. You can read the tutorial on Numerical Calculations with Units, which also provides an example:

FindRoot[1/
  2 (t^2 (Quantity[-9.8, ("Meters")/("Seconds")^2]) + 
    Quantity[200, "Meters"]), {{t, Quantity[1, "Seconds"]}}]

(* {t -> Quantity[4.51754, "Seconds"]} *)
POSTED BY: Domen V
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