Исходники иллюстрации

\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}
    \draw[interface] (-3,2) coordinate (lb) rectangle ++ (6,0.5);
    \draw[thick] (lb) -- ++(6,0);
    \draw[magenta, fill=magenta!10] (0,0) circle (2cm);
    \draw[magenta, fill=magenta] (0,2) circle (2pt);

    % \draw (0,-2.5) node[blue]{$\bigotimes$} node[right] {$\vec{v}$};
    \draw[axis] (-3,0) -- (3,0) node [right] {$+x$};
    \draw (0,0) node[magenta, scale=1.35]{$\times$};

    \begin{scope}[yshift=-2cm, xshift=0cm]
        \lineann[4cm]{90}{4}{$l$};
    \end{scope} 

    \begin{scope}[yshift=0cm, xshift=0cm]
        \lineann[3.25cm]{-90}{2}{$R$};
    \end{scope}     

    \draw[magenta, fill=magenta!100] (1,0) -- ++(0, 1.73) coordinate (o) -- ++ (-0.1,0)  coordinate (I) -- ++ (0,-1.73)  coordinate (II) -- cycle;

    \draw[axis,->] (4,-1) -- ++(0,3.2) node[above] {$+y$};

    \hrLabel{4}{0}{$0$};
    \hrLabel{4}{1.73}{$y(x)$};
    \draw[axis] (o) -- ++ (3.2,0);
    \draw[blue,<-] (0.9, -0.1) -- ++ (-0.3,0);
    \draw[blue,<-] (1, -0.1) -- ++ (0.3,0);
    \draw[blue] ($(0.9, -0.1)!0.5!(1, -0.1)$) node[below, yshift=-0.3em] {$dx$};



\end{tikzpicture}\end{document}