(a) Show that for Poiseuille flow in a tube of radius \(R\) the magnitude of the wall shearing stress, \(\tau_{r z}\), can be obtained from the relationship

\[ \left|\left(\tau_{r z}\right)_{\text {wall }}\right|=\frac{4 \mu Q}{\pi R^{3}} \]

for a Newtonian fluid of viscosity \(\mu\). The volume rate of flow is \(Q\).

(b) Determine the magnitude of the wall shearing stress for a fluid having a viscosity of \(0.004 \mathrm{~N} \cdot \mathrm{s} / \mathrm{m}^{2}\) flowing with an average velocity of \(130 \mathrm{~mm} / \mathrm{s}\) in a 2-mm-diameter tube.