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msuta1-Visitor
1-Visitor
August 9, 2018
Question

Only raise value to power and not units

  • August 9, 2018
  • 2 replies
  • 3666 views

Hi,

 

I was wondering if there is an option to program when in a formula I can raise to power only the numeric value and not the units. I hope this makes sense.

 

I couldn't find any similar situation in the forum. Please see photo inserted below:

 

 

I would like to be able to use m (from photo) in a formula but only raise to the 2/3 power the number and not the units.

 

There's probably an easy fix and I can't find it.

I have attached a file with the example in the snapshot.

 

Thank you

M

2 replies

L
LucMeekes23-Emerald IV
23-Emerald IV
August 9, 2018

There is a way:

LM_20180809_Units.png

 

But you should only use it when you know exactly what you are doing.

Generally any united value, raised to a power, should give meaningful resulting value with a meaningful unit.

This should be the standard, and applies to equation describing (physical) phenomena that are derived from general theory.

However some empirical formulae use fudge factors and constructs that may require that you strip off units.

As said, handle with care.

 

Success!
Luc

 

P.S. Your photo didn't come through. The functionality of this forum still leaves wishing for.... it to be up to standards. Yes, you can copy and paste graphics into an edit window. But if you press the post button the graphics are lost. To insert a picture you have to have the picture saved as a file, and use the 'photocamera' icon that pops up a window where you can select and embed the picture.

 

 

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
August 9, 2018

Usually (not always) when you're raising a unit connected variable to a non-integer power you're applying an empirically derived equation from an engineering manual (an OLD manual!)  There is almost always a numerical coefficient in that formula.  The units of the values in the formula are specified. ("length in feet, speed in rpm.").  It's possible some times to assign units to the undefined numerical coefficient.  (There are examples in this forum of people wondering why the formula for power in horsepower doesn't work; because they have a coefficient they no longer need because Mathcad understands units.

 

Luc's example assumes that you want to raise length (meters) to the 2/3 power and report the result in units of length.  That may be what you want, but it makes no physical sense. 

 

Each of these occurrences is a special case and may require its own solution.

 

Good luck

W
Werner_E25-Diamond I
25-Diamond I
August 9, 2018

Mathcad does not remember that you defined m in kN. The variable is stored in its base unit.

So Prime cannot provide a way strip the unit kN you used when defining m. You have to do it yourself and you also would have to add that very unit manually yourself after the calculation.

Using "SIUnitsOf" or "UnitsOf" will not give you the result  you expect.

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