- #1

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[tex] \tau = \mu \frac{du}{dy}[/tex]

Now, let's assume you have water flowing horizontally in laminar steady state motion, where its surface is also moving. Above it is air. So if we disregard the friction from air, the shear-stress will become zero for the surface.

But why? the formula given above is not even defined for the surface layer, and besides you have shear-stress from the water below the surface as well... So what's up?