Units system nuissance

[dBoggs]

It seems that all of the various systems of units Mathcad offers, they are all "absolute," i.e., mass is a primary unit and force units are derived from it. Nearly all of my work uses a "gravitational" system in which force is a primary unit and mass is derived from that. Most commonly I use lbf (although I would prefer to label it "lb" as the force unit and slugs or lb-mass (which I would prefer to label "lbm" as a derived mass unit.

The nuisance arises because, in any calculated result including the unit of force, Mathcad insists of displaying it in units of mass. For example, if I define F1 := 10*lbf and then ask Mathcad to display 2*F1 = ? it will give me the answer 2*F1 = 20 slug-ft/s^2. I have to convert this to display 2*F1 = 20 lbf. I have to do this all the time and I get tired of it.

Is there some way to change this behavior? I'd like to define a custom units system, but apparently I am limited to using mass as a primary unit and I want to use force.

[ValeryOchkov]

Mathcad or Mathcad Prime?

We can use customer units (system) in Mathcad 12(?)-15 but only from the built-in list. I do not see slug-ft/s^2 as the force unit in this list.

[dBoggs]

Sorry, thought the question probably applied to all versions. I am on M15.

[ValeryOchkov]

I have same problem in my thermal calculation and solve it so - see the pictute and attach.

[dBoggs]

Thanks Valery, I can see your problem, but am not sure about your "solution". I know that you can get mcad to show the desired units by defining the unit and then typing it in after mcad shows the default units. So I guess you're confirming the issue.

Maybe this is just a request to mcad for a new feature: allow me to define a custom units system having force as a primary unit and mass as a derived unit.

[ValeryOchkov]

I think that such problems (with force as a primary unit and mass as a derived unit) do not have much to ask for a new feature. But... Can you explain to me / us why your task requires force as a primary unit and mass as a derived unit. Habit - is second nature!

For example in my thermal task I try to use J/(g K) (Brevity - the soul of wit) not kJ/(kg K) - but... Habit - is second nature!

[dBoggs]

It's because, in my work (structural engineering), I am always dealing with forces, or stress (force/area). A small fraction of the work involves mass.

So the results of most calculations are force, or something involving force, but rarely involving mass.

In an absolute units system, the default display of a force is mass / g. I don't want that because I have to manually convert it to lbf. I want the default display to be in lbf, the force unit.

It's not unusual to have a mathcad file with 100 formulas to evaluate, all of which contain force not mass. They need to display in lbf, and I have to manually convert every one of them!

[Fred_Kohlhepp]

Just a reminder, I'm sure you know, when you type the equals key to evaluate an expression if you then hit the "tab" key (in Mathcad 15 and before) the cursor will jump to the unit placeholder. Thus the key sequence:

"="

"tab"

"lbf"

will change your "slug ft/sec^2" to lbf.

That's about as short as you can make it.