How to make plot of non linear equation with Maple?

I have differential equation : $x"=r*(y')^2-g/r^2$, and $y"=-2*y'*x'/r$, by $r$ and $g$ are constant. How to make plot of this non linear equation with Maple?

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 What are $x"$ and $y"$? Did you mean $x''(t)$ and $y''(t)$? Do you want the plot of possible solution? If so, you should consider initial conditions here and more importantly the values of constants should be given. :-) – Babak S. Jun 21 '12 at 17:58