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MelB3-Newcomer
3-Newcomer
December 13, 2023
Solved

There is no keyword named [_], Solving unknown variable

  • December 13, 2023
  • 1 reply
  • 1094 views

See snip below & attached. I'm trying to solve for z_pna, such that the sum of the tension forces equals the sum of the compression forces.

MelB_0-1702496329351.png

 

Best answer by LucMeekes

The variable you want to solve for needs to be part of the equation. That is not the case above. You should have z_pna be a parameter to both  SigmaT and SigmaC. This in turn means that also Tsr and Csrw must be functions, with z_pna as a parameter.

Considering that all other items are numerically defined, there's no need to try to solve this symbolically (which you do when you use that arrow). Better try a numerical solver, such as the root() function.

And when you really do want to use the symbolic solver:

1. You need to use the 'solve' keyword.

2. You should consider that the symbolic processor does not know about units by default, and while there is a setting to let it take account of them, sometimes it simply doesn't work.

 

Success!

Luc

1 reply

L
LucMeekes23-Emerald IVAnswer
23-Emerald IV
December 13, 2023

The variable you want to solve for needs to be part of the equation. That is not the case above. You should have z_pna be a parameter to both  SigmaT and SigmaC. This in turn means that also Tsr and Csrw must be functions, with z_pna as a parameter.

Considering that all other items are numerically defined, there's no need to try to solve this symbolically (which you do when you use that arrow). Better try a numerical solver, such as the root() function.

And when you really do want to use the symbolic solver:

1. You need to use the 'solve' keyword.

2. You should consider that the symbolic processor does not know about units by default, and while there is a setting to let it take account of them, sometimes it simply doesn't work.

 

Success!

Luc

W
Werner_E25-Diamond I
25-Diamond I
December 14, 2023
@LucMeekes wrote:

2. You should consider that the symbolic processor does not know about units by default, and while there is a setting to let it take account of them, sometimes it simply doesn't work.

 

How true! In case of this example it works OK,  though. You might have to recalculate the sheet once or twice.

Werner_E_0-1702568138327.png

 

But I fully agree that if just a numeric result is looked for, a numeric method like a solve block or the root function should be used, rather than the unit-unaware symbolic "solve".

 

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