\documentclass[a4paper]{article}% Dominique Rodriguez/hv 2005 \usepackage[T1]{fontenc} \usepackage{lmodern} \usepackage{pstricks,pstricks-add} \pagestyle{empty} \begin{document} \newcommand{\InitCond}{ 0 0 5 10} \newcommand{\OMEGA}{6.283185} \newcommand{\h}{0.1} \psset{unit=2} \begin{pspicture}[showgrid=true](0,-2)(3,1.2) \psset{method=rk4,algebraic=true,whichabs=0,whichord=1} \psplotDiffEqn[linecolor=red,linewidth=1pt,plotpoints=500] {0}{10}{\InitCond}{y[2]|y[3]|\OMEGA*y[3]-\h*(sqrt(y[2]^2+y[3]^2))*y[2]|-\OMEGA*y[2]-\h*(sqrt(y[2]^2+y[3]^2))*y[3]} \psline{->}(0.5,1) \uput[90](0.5,1){$\overrightarrow{v}_0$} \pscircle{0.1}\psdot(0,0) \uput{3mm}[180](0,0){$\overrightarrow{B}$} \end{pspicture} \end{document}