  \begin{pspicture}*(-4.5,-4.5)(4.5,4.5)
      \psgrid[griddots=10, gridcolor=gray, subgriddiv =0](-5,-5)(5,5)

	  \psdot(0,0)
	  \uput[u](0,0){$O$}

	  \psline{->}(0,0)(0,-3)
	  \uput[l](0,-1.5){$\vec{F_3}$}
	  \psline{->}(0,0)(2,1)
	  \uput[u](1,0.5){$\vec{F_1}$}
	  \psline{->}(0,0)(-2,2)
	  \uput[u](-1,1){$\vec{F_2}$}


  \end{pspicture}
