= Wall shear stress
{title2=$\tau_w$}
For a stationary flat plate in a <Newtonian fluid>, the streamwise <shear stress> exerted by the <fluid> on its upper face is
$$
\tau_w=\mu\left.\frac{\partial u}{\partial y}\right|_{y=0}.
$$
The sign here is positive when the <fluid> pulls the plate downstream; the plate exerts the opposite tangential <traction> on the <fluid>. In a <Blasius boundary layer>, $\tau_w\simeq0.332057\rho U^2/\sqrt{\mathrm{Re}_x}$. A growing <boundary layer thickness> therefore accompanies decreasing local <wall shear stress>, because its near-wall <velocity> <gradient> becomes gentler. The formal divergence as $x\to0$ is integrable, but that leading-edge limit lies outside the thin-layer hypothesis.<Modules on High Reynolds Number Flows/Boundary-layer Thickness, Skin friction, and Energy dissipation>{ref}
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