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dlr1-Visitor
1-Visitor
November 8, 2014
Solved

Units with exponentiations

  • November 8, 2014
  • 3 replies
  • 2320 views

Hi,

I'm having trouble with units when using exponentiated variables. I'm trying to assign a variable a value (fck) while having another variable depend on the first one (fctm). It works fine if I only multiply the first variable with a constant, but if I also try to exponentiate the first variable my units (MPa) get messed up (see fctmERROR). However if I replace the first variable with the numerical value I assigned to it my units works as they should (see fctmWORKING).

Trying to forcibly remove the messed up unit only breaks the whole thing (fctmERROR2).

I have attached an image to better understand the problem:

Units_with_exponentiations.jpg

Also attached is a file with the error in question.

Im using MathCAD Express Prime 3.0 F000 (2013.09.12.003/3.0.0.194) running on Win 8.1 x64

Best answer by ViktorKorobov

I see no errors. If f has units Mpa then f^(2/3) has units Mpa^(2/3).

Mpa.png

Viktor

3 replies

V
ViktorKorobov1-VisitorAnswer
1-Visitor
November 8, 2014

I see no errors. If f has units Mpa then f^(2/3) has units Mpa^(2/3).

Mpa.png

Viktor

W
Werner_E25-Diamond I
25-Diamond I
November 8, 2014

The only error I see in your picture is the attempt to evaluate a variable (fctmERROR2) which was never assigned a value and thats exactly what Prime tells you.

You don't tell us why you think that Prime is messing up your units.

Which units do you expect to see? MPa as in the last expression you show? That would be wrong!

Did you expect to see MPa^(2/3)? Then you will have to replace the unit displayed by Prime with that expression yourself as Viktor had done in the last line of his picture.

Prime is correct with its units and you have to consider that 1Pa=1kg/(m*s^2) in base units. Thats what Prime switches to when an expression gets too complicated (1MPa ^2 is displayed simply in MPa^2).

There is no way to simply tell Prime to show MPa, you have to supply the full resulting unit. If you simply type MPa for the unit, Prime will replace the rest with kg, m and s.

An easier example: let A:=20mm * 30 mm. When you evalaute A you will get the result in base units, so you get square meter. If you simply type mm for the unit you will see the result as 0.6 m * mm. YOu have to type te full mm^2 to see 600 mm^2.

D
dlr1-VisitorAuthor
1-Visitor
November 8, 2014

Thank you for your answers.

I think I see where my fault lies now. Since fck already contains the unit MPa then exponentiating that variable will also exponentiate the unit MPa, I wrongly assumed that exponentiating fck would only affect the value and not also the unit.

The unit I was expecting to see was MPa, like I had done with fctmWORKING.

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