Spo�t�te, �e $\int \! \BBb F {d \vec S} = 0$ pro 
$\Bbb F(x,y,z) = (y,-x,0)$ a \\
$S= \{(x,y,z) \in \Bbb R ^3$, $x^2 + y ^2 \le 9$,
$z = \sqrt {x^2 + y^2 } \}$. 
