Исходники иллюстрации

\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}
    \draw[fill=black!30] (0,0) rectangle (4,2);
    \draw[fill=white] (0.25,0.25) rectangle (3.75,1.75);
    \draw[pattern=north west lines, pattern color = blue] (1.9,0.25) rectangle ++(0.25,1.5);

    \draw (1,1) node {$1$};
    \draw (3,1) node {$2$};

    \draw[force,->] (2.15,1) -- ++ (0.5,0);

\end{tikzpicture}\end{document}