\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}
