5

Engineering texts often use a straight or curved arrow paired with a "plus" sign to indicate the sign convention for forces and moments at the beginning of an equilibrium equation. For example, see the below image from an engineering textbook, where I have circled in red three of these symbols:

My experience has been that these symbols are standard and common but I can't find a way to create them using any TeX/LaTeX markup or library. Do I have to somehow create them from scratch? Most of my familiarity with TeX and LaTeX comes from using MathJax here on Stack Exchange.

  • Welcome to TeX - LaTeX! There are many arrow symbols available. Have you tried the resources in How to look up a symbol or identify a math alphabet?? – Andrew Swann Jan 11 '17 at 18:39
  • @AndrewSwann Thank you. I did use Detexify and came up blank. The lengthy indices are only helpful if you know which section to look in or have useful keywords for searching. The pertinent keywords for this search are in the question title and did not help me find a symbol in those indices. – Air Jan 11 '17 at 18:44
7

You can combine existing symbols.

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{graphicx}

\makeatletter

\newcommand*\curveplus{%
  \mathbin{\rotatebox[origin=c]{90}{$\m@th\curvearrowleft$}+}}

\newcommand*\rightplus{%
  \mathpalette\@rightplus\relax}
\newcommand*\@rightplus[1]{%
  \mathbin{\vcenter{\hbox{$\m@th\overset{#1+}{\to}$}}}}

\newcommand*\upplus{%
  \mathbin{+\mathord\uparrow}}

\makeatother

\begin{document}


$\curveplus \sum M_A = 0$

$\rightplus \sum F_x = 0$

$\upplus \sum F_y = 0$

\end{document}

enter image description here

  • 1
    That looks pretty good. Which commands are from graphicx - the rotate and overset? – Air Jan 11 '17 at 18:38
  • 2
    @Air \rotatebox is from graphicx and \overset is from amsmath. – Henri Menke Jan 11 '17 at 18:40
  • Your edit introduced several new elements. This answer could be improved by explaining their purpose. – Air Jan 13 '17 at 16:58
  • What is the \m@th command? – gigabytes Jan 23 '17 at 22:34
  • @gigabytes It sets \mathsurround=0pt. – Henri Menke Jan 24 '17 at 6:53
3

If you want MathJax-friendly solutions, you need to accept either verbosity or approximations.

First, the approximate solutions, which are pretty compact (and some were given in @HenriMenke's original answer):

  • Positive counter-clockwise moment: \overset{\curvearrowleft}{+} counter-clockwise moment with arrow above plus-sign
  • Positive rightward horizontal force: \overset{+}{\rightarrow} positive horizontal force
  • Positive upward vertical force +\!\!\uparrow positive vertical force

I personally find the force symbols perfectly acceptable. The problem is the moment one, which displays the arrow above the plus sign, not beside it as is customary.

To render that symbol correctly, you need to go big or go home:

${\style{display: inline-block; transform:rotate(0.75turn);}{\curvearrowleft}}\!+$ is rendered as correct positive counter-clockwise moment, which I once again think is quite satisfactory.

Now, if you're not in inline mode and have large symbols (such as sums), the symbols won't work too well because the alignment is off, so more work is needed:

$${\style{display: inline-block;
        transform:rotate(0.75turn);
        position: relative;
        top:2.5px}{\large\curvearrowleft}}
\!{+}\! \sum M_A = 0$$

$${\style{display: inline-block;
        position: relative;
        top:7px}{\overset{\Large+}{\rightarrow}}}
\sum F_x = 0$$

$$+{\uparrow}\!\sum F_y = 0$$

enter image description here

You'll notice there are a few changes compared to what was given before. Specifically, a term position: relative; top:2.5px was added to the moment style. This is because the large summation symbol increased the equation's height, so we have to adjust the height by hand (simply adding vertical-align:middle doesn't work as well, in my opinion).

For the same reason, the equation for horizontal force also had to get the \style treatment. Here the required offset is even greater (vertical-align:middle looks quite terrible in this case) and I also made the plus-sign larger.

I also made liberal use of negative space \! to tighten up the symbols, feel free to add more or remove them as you see fit.

This also got me curious as to how to draw the classic internal force convention symbol:

internal force convention symbol Source

And this is the best I could do (and it really isn't much):

$${\longleftarrow}
\!\!\!\!\!\!\!\!\!\!
{\Large{\style{display: inline-block;
               transform:rotate(0.75turn);
               position:relative;
               top:1px}{\curvearrowright}}}
\!\!\!\!
\uparrow
\!
\boxed{+}
\!
\downarrow
\!\!\!\!
{\Large{\style{display: inline-block;
               transform:rotate(0.25turn);
               position:relative;
               top:1px}{\curvearrowleft}}}
\!\!\!\!\!\!\!\!\!\!
{\longrightarrow}$$

enter image description here

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