Hello, I read some fluidmechanics and there was something I didn't understand. The shear stress in a newtonian fluid is tau=viscosity*dV/dy, (no need to be dy, but dx and dz also can do.) A shear component called tau(xx) came up, I have two questions about this component: 1. Shear is supposed to be parralell on a surface, so how does this shear component work? How can it point in the x-direction, when it is on the x-surface(yz-plane) and also is supposed to be in the yz-plane? 2. It is said that in a Newtonian fluid tau(xx)=2*viscosity*du/dx, where the velocity in the x-direction. Why is it this, why the number 2?, can you explain this if you look at the definition of viscosity in Newtonian fluids I posted first? Then my question is about the stress component tau(xy). It is said that it is viscositu*(du/dy+dv/dx). I can see out of the definition that it is supposed to be viscosity*du/dy, but why also the dv/dx part?(v is the y-compononent of the velocity). These questions have been nagging me for ours now, I would appreciate some help. PS: All the deriviatives are supposed the be partial deriviatives offcourse.