How should I typeset the physical dimensions of quantities?

Question

As an example: The Newton's Law gives

F=ma      vs.      dim F=MLT^{-2}

My question is: Should I typeset the "MLT" in \mathit shape?

You can contrast

\[\mathit{LT}\mathit{LT}\mathit{LT}\mathit{LT}\]
\[\mathit{L}\mathit{T}\mathit{L}\mathit{T}\mathit{L}\mathit{T}\mathit{L}\mathit{T}\]

the latter is more close together. Is there some tricks on using \mathit shape?

Or I should choose the \mathrm family, or even just the default math font?

Answer 1

Both, the SI-Brochure nr. 8 (section 1.3, p. 105) and DIN EN ISO 80000-1:2012-10 (chapter 5, p. 19) use upright capital and sans-serif letters for this.

Quantity symbols are always written in an italic font, and symbols for dimensions in sans-serif roman capitals - SI-Brochure

Therefore you should write dimensions of quantities like this:

% arara: lualatex

\documentclass{article}
\usepackage{mathtools}
\usepackage[%
    ,math-style=ISO
    ,bold-style=ISO
    ,sans-style=italic
    ]{unicode-math}

\begin{document}
\[\dim Q = \mathsfup{L}^\alpha \mathsfup{M}^\beta \mathsfup{T}^\gamma \mathsfup{I}^\delta \mathup{\Theta}^\varepsilon \mathsfup{N}^\zeta \mathsfup{J}^\eta\]
\end{document}

This yields:

Unfortunately, there is no sans-serif font for medium-weight Greek letters in the range of uni-code characters. But if you normally use italic capital Greeks in your document (math-style=ISO), it will be distinguishable.

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Three other remarks on your OP:

• Your two examples for $\mathit{LT}$ and $\mathit{L}\mathit{T}$ should (and do) look the same.
• The default math font normally yields the same as \mathit{...}.
• Before I edited your question, you wrote: "[F]=MLT^{-2}". Please note that [Q] means "the unit of Q". {Q} would signify "the value of Q" and for your case, it should be dim Q. $Q = \{Q\}\cdot[Q] \wedge \dim Q = \mathsfup{L}^\alpha \dots$.