Getting MathCAD to stop cancelling some units?

Question

MathCAD is too good. It's doing more than I really want it to do, particularly with some units.

Today's problem is the pound force. MathCAD knows the definition of the pound force, and the problem is that when I want to keep something as a pound force, it instead inserts the definition of the pound force and cancels out everything that can be cancelled out. When I try multiplying out by the unit I want, it still does the same thing.

I'm trying to use the spring constant k in my US standard formulae. Normally, k=force/distance, or lbf/ft. Since the definition of the pound-force includes feet (I think it's something like ft*lb/s^2), then MathCAD automatically cancels out the feet and leaves me with s^2/lb, which makes no sense in the context of what I'm trying to do.

How can I get MathCAD to just mind its own business and leave lbm as lbm? I can't seem to find a way to redefine the built-in lbm and I'm not certain I want to do that anyways.

(I understand that this sort of topic is well-discussed; I've tried hunting around on this forum and others and can't seem to find the answer. Maybe my Google-fu is weak)

DJF · Accepted Answer

Yes, Mathcad will often give you some undesirable units in the result.  in-lbf is a common one for me also; it is not the mathcad default unit.  However, all you have to do it type over the units it gives you with the units you desire.  (It will give you those typed units plus anything else it needs to make the units work, so if it's adding additional units you've formulated something wrong.)  If that doesn't work please post an example.  But I use lbf and lbm all the time with no issue (Prime anyway, I assume 15 is the same)

LucMeekes · Answer

Mathcad internally works with SI units, no matter what unit system you choose to work with.

Ah, that's not true. Mathcad internally works with units of time (1T), length (1L), mass (1M), charge (1Q), temperature (1K), substance (1S), luminous intensity (1C) and money (1$).

They are by default defined as SI units (1T = 1 second, 1L = 1 metre, etc.), apart from 1$.

The only way you can control somewhat the behaviour of mathcad's working with units, is by telling it to not use any unit system. Then you can, no, have to, define your own unit system. The downside of this is that you have to tell mathcad with most every result what the unit is you want it to be displayed in. Basically that is the same as what you can do now: without defining your own unit system, but using a built-in one, just fill the units-placeholder of a result with the unit you want in there.

In short: you've searched and not found for good reasons: there is no way of forcing mathcad into using the units of your choice in its results other than to set those units (every time...).

Luc

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