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ptc-51285311-Visitor
1-Visitor
June 2, 2013
Solved

Mathcad 15 gets confused when solving equations with units in them.

  • June 2, 2013
  • 1 reply
  • 10926 views

Why is it that mathcad is having trouble solving with units? Normally when I want to make a calculation with alot of units, it is no problem, or if I solve the equation my self and then hit the equal sign, there are no problems.

But when i use the solve or given/find function I will have to copypaste the result, remove alle the units, and then it works? It is like it gets confused with all the units

Best answer by Werner_E

Humptey Dumptey wrote:

I can write my equation, goto symbolics>variables>solve, and then it rewrites the expression below.

IMHO thats not the best method using the symbolics as what you do is a static evaluation. Changing the input equations won't change the result and you will have to do a lot of copy and paste which is rather failure prone and does not result in a universally useable worksheet. Better to use the dynamic evaluation.

The symbolic processor does not know anything about units, it treats them as unknown variables. If you follow the symbolic evaluation by a numeric one (the equal sign), the numeric processor will take care of the units, cancel them or whatever will be necessary. You can do this on the result of the dynamic symbolic evaluation (the one with the arrow) as well, but there may be better methods.

Attach you worksheet (upper right, chose "use advanced editor") to let us see your equations.

1 reply

W
Werner_E25-Diamond I
25-Diamond I
June 2, 2013

Do you use the symbolic "solve" or are you talking about the numeric solve-block using "find".

Unfortunately both may have their problems with units but if you post your worksheet there may be a workaround which at least does not utilize copy and paste.

On possible cause may be that your solution vector contains values with different units which is a no go for Mathcad 15 and below. In that case usually there is an easy way out, though.

P
ptc-51285311-VisitorAuthor
1-Visitor
June 2, 2013

I can write my equation, goto symbolics>variables>solve, and then it rewrites the expression below. If I i do the equal sign on that new expression it works fines with units.

But if i just use the solve button in the "symbolic" window so it looks like "solve,x->" it is not happy with units. Also if I do the given/find it will not calculate.

W
Werner_E25-Diamond IAnswer
25-Diamond I
June 2, 2013

Humptey Dumptey wrote:

I can write my equation, goto symbolics>variables>solve, and then it rewrites the expression below.

IMHO thats not the best method using the symbolics as what you do is a static evaluation. Changing the input equations won't change the result and you will have to do a lot of copy and paste which is rather failure prone and does not result in a universally useable worksheet. Better to use the dynamic evaluation.

The symbolic processor does not know anything about units, it treats them as unknown variables. If you follow the symbolic evaluation by a numeric one (the equal sign), the numeric processor will take care of the units, cancel them or whatever will be necessary. You can do this on the result of the dynamic symbolic evaluation (the one with the arrow) as well, but there may be better methods.

Attach you worksheet (upper right, chose "use advanced editor") to let us see your equations.

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