\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usepackage{blindtext}
\begin{document}
Usual spacing: $ \dfrac{1}{2} \cdot a \cdot t^{2} $
Wrong spacing: $ \dfrac{1}{2} \cdot \tikz[remember picture,overlay]{\node[baseline=(A.center),above,inner sep=0pt] (A) {$a$};} \cdot t^{2} $
\blindtext
$ F = m \cdot \tikz[remember picture,overlay]{\node[baseline=(B.center),inner sep=0pt,above] (B) {$a$};} $
\tikz[remember picture,overlay]{\draw[blue, <->] (A) to [bend left] (B);}
\end{document}