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## Main Question or Discussion Point

Can anyone tell me why this is true? I can't find an explanation anywhere, and it doesn't make sense to me geometrically either, especially for i=j. Isn't viscous force, by definition, a

[tex]\mathbb{T}_{ij} = \mu\left(\frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i}\right)[/tex]

**shear**force? How can it produce a**normal**stress? Also why would Txy=Tyx? Why can't they be different, producing a net moment on the volume element? What says that can't happen? I'm getting extremely frustrated.[tex]\mathbb{T}_{ij} = \mu\left(\frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i}\right)[/tex]

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