= Solution
Here $u=v=x-y$. The <streamlines> are \b[$x-y=\text{constant}$], straight lines parallel to $(1,1)$. The entire line $x=y$ is stationary. On the $x-y>0$ side the motion is northeast, and on the $x-y<0$ side it is southwest. In orthonormal coordinates $p=(x+y)/\sqrt2$, $q=(x-y)/\sqrt2$, the velocities are $\dot p=2q$, $\dot q=0$: this is <simple shear flow>. The right panel shows both moving families and the stationary line.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2014/ib/paper-2-streamlines.png]
{title=Planar strain, counterclockwise rigid rotation and diagonal simple shear, with arrows showing the three streamline directions}
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