\documentclass[10pt,multi=False,border=5pt,tikz,class=scrartcl]{standalone}
\usepackage[latin1]{inputenc}
\usepackage{xcolor}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usetikzlibrary{calc,arrows,arrows.meta,shapes.arrows}
\usetikzlibrary{decorations.pathmorphing,patterns,decorations.markings}
\usepgfplotslibrary{fillbetween}
\usepackage{lmodern}
\renewcommand*{\familydefault}{\sfdefault}
\begin{document}
\begin{tikzpicture}[>=latex',
every node/.style={scale=1.1}]
\draw [fill=brown] (0,0) rectangle (8,-.5);
\draw [color=blue!80!black, fill=cyan, rounded corners=8pt]
(1,.2) circle (.2) (2,.2) circle (.2) (.5,.4) rectangle (2.5,1.5)
node [pos=.5] {\color{black}$\mathrm{m}_\mathrm{1}$}
(5,.2) circle (.2) (6,.2) circle (.2) (4.5,.4) rectangle (6.5,1.5)
node [pos=.5] {\color{black}$\mathrm{m}_\mathrm{2}$};
\draw [thick,red,->]
(2.5,.95) -- ++(2,0) node [pos=.5,above] {string}
(6.5,.95) -- ++(1.5,0)
node [pos=.5,above] {\color{black}$\mathbf{F}_\mathrm{ext}$};
\node [text width=10em,align=center] at (2,-1.25)
{$\begin{aligned}
\mbox{Not forces:}&\tikz[baseline=-.5ex]\node[draw,single arrow,text width=2em,scale=.4] {};& \mathrm{m}_1\mathbf{a}\\
\mbox{Forces:}&\tikz[baseline=-.5ex]\draw[->,very thick] (0,0) -- (++.5,0);& \mathbf{F}_{21}
\end{aligned}$};
\node [text width=10em,align=center] at (5.5,-1.35)
{$\begin{aligned}
\tikz[baseline=-.5ex]\node[draw,single arrow,text width=2em,scale=.4] {};& \mathrm{m}_2\mathbf{a}\\
\mathbf{F}_{12}&\tikz[baseline=-.5ex]\draw[<-,very thick] (0,0) -- ++(.5,0);\\
\tikz[baseline=-.5ex]\draw[->,very thick] (0,0) -- ++(.5,0);&\mathbf{F}_\mathrm{ext}
\end{aligned}$};
\end{tikzpicture}
\end{document}