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prop_design15-Moonstone
15-Moonstone
August 23, 2020
Solved

classic mathcad issue with lbf in plots

  • August 23, 2020
  • 3 replies
  • 4370 views

hi,

 

i noticed a weird thing, possibly a bug, with classic mathcad. i found a trick reading through old posts. i'm using the trick now. however, i'm wondering if others have observed this.

 

if you try to plot force in units of lbf by switching the unit system to US, the program plots Force*gravity instead. if you take what mathcad plots and divide by g, then it will show force in units of lbf.

 

the trick i found on here was to just use the metric system and divide force by lbf. that works good. so i'll use this going forward. however, this seems like a bug when trying to use the built in US unit system. the program outputs any force variable in lbf. it just plots that variable in lbf * g instead of plotting lbf.

 

anthony

    Best answer by Werner_E

    To further clarify:

    Werner_E_0-1598272758523.pngWerner_E_1-1598272800924.pngWerner_E_2-1598273118475.png

    I still tend to call this a bug (which is not present in Prime)

     

    3 replies

    T
    tietjee15-Moonstone
    15-Moonstone
    August 23, 2020

    Could add the link to the old post?

    P
    prop_design15-MoonstoneAuthor
    15-Moonstone
    August 23, 2020

    sorry, i'm not entirely sure which post mentioned the work around. however, the work around wasn't related to the bug. it was similar enough to allow me to get what i wanted though.

     

    i figured i should post this to see if anyone else has noticed it. it doesn't seem right. i tried creating a custom unit system (based off the US system) and it just made things even weirder. you can't control the force output with mathcad. it just allows it's built in types. but if you change length and time then the plot output for force is even more messed up. it takes your length and time into account and also multiplies by gravity. so the bug aspect is in the unit definition it says pound force. it will display that for a variable but not in the plot. the other aspect of the bug is this is only for the built in US system or a customization of it. the metric systems don't have this behavior.

     

    so using the help from the old post, i can get what i want. which was velocity as the x axis in mph and force in lbf for the y axis. i leave the program in SI units. then plot F/lbf and v/mph. then the output of the plot is what i want.

    W
    Werner_E25-Diamond I
    25-Diamond I
    August 23, 2020

    Dividing a physical quantity by the unit you'd like to see in the plot is standard behaviour in classic Mathcad (and is independent from the unit system selected).

    Nonetheless is the effect you describe somewhat strange and I also tend to call it a bug.

     

    F
    Fred_Kohlhepp23-Emerald I
    23-Emerald I
    August 24, 2020

    I don't have "classic" Mathcad any longer, so I can't verify what you're saying, and while I worked (almost) exclusively in English units, I never bothered to change the default from MKS.

     

    I would note that Mathcad would recognize "lb" and "lbm" both as mass units (equivalent), if you wanted force it had to be "lbf."  Whether that's the source of your issue or not I can't speculate.

    FredKohlhepp_0-1598240757583.png

     

    P
    prop_design15-MoonstoneAuthor
    15-Moonstone
    August 24, 2020

    hi fred,

     

    no i'm afraid the issue is different. it has to do with the magnitude of force in a plot. the built in unit system says pound force is the output. however it plots lbf * g. if you make a custom unit with 'US' as the starting system, it does the same thing. even worse, if you change the units of distance and time it also adds that in. so the magnitude of the force that is plotted can be really messed up. it definitely seems to be a bug, as this doesn't happen if you use any of the metric systems. moreover, it doesn't plot N * g.

     

    awhile ago i tried prime and didn't like it. so i'm not sure what prime does or how it works. honestly, i don't care at all. one thing that pissed me off with prime is i made a handful of worksheets and decided to switch back to classic. there was no way to convert the files. i had to manually recreate them all. they were simple files. there was no reason they couldn't work in classic mathcad or prime. ptc just doesn't want you to switch back. they basically wasted a lot of my time, which i don't appreciate. so i have no intention of trying it out again. the main thing i like about classic mathcad is the help system and all the built in worksheets.

     

    i've used mathcad on and off since the 90s. this is the first time i ran into the problem of plotting force. i haven't used mathcad in a long time. so i don't really remember how i worked with it way back when. i was surprised to find this issue. i was mainly curious if anyone else has noticed it.

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