$5\mathrm{.}1$ Demonstrate that the viscous forces acting on an element of incompressible fluid of volume unity can be expressed in terms of the rotation by the following expression:

$-2(\frac{\text{d}\text{e}\text{l}}{\text{d}\text{e}\text{l}\text{y}}-\frac{\text{d}\text{e}\text{l}}{\text{d}\text{e}\text{l}\text{z}})$

The two other expressions are obtained by permutation.