\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}
\draw[interface] (-1,0) rectangle ++(4,-0.25);
\draw[thick] (-1,0) -- ++(4,0);
\fill[magenta!10, draw=none] (0,0) rectangle ++(2,2.75);
\draw (0,3.2)--(0,0)--(2,0)--(2,3.2);
\draw[white] (2,0.5) -- ++(0,0.4);
\draw[white] (0,1.5) -- ++(0,0.4);
% Draw line annotation
% Input:
% #1 Line offset (optional)
% #2 Line angle
% #3 Line length
% #5 Line label
% Example:
\begin{scope}[yshift=0.7cm]
\lineann[-2]{90}{1}{$\Delta h$}
\end{scope}
\fill[magenta!10] (-0.4,1.5) rectangle ++(-0.4,0.4);
\fill[magenta!10] (2.8,0.5) rectangle ++(-0.4,0.4);
\draw[force,->] (-0.6,1.7) -- ++ (-1,0) node[left] {$\vec{v}_1$};
\draw[force,->] (2.6,0.7) -- ++ (1,0) node[right] {$\vec{v}_2$};
\draw[force,->] (0,1.7) -- ++ (1,0) node[right, above] {$\vec{F}_{r1}$};
\draw[force,->] (2,0.7) -- ++ (-1,0) node[left, below] {$\vec{F}_{r2}$};
\draw[axis] (-1,-0.5) -- ++(4,0) node[right] {$+x$};
\end{tikzpicture}\end{document}
