Could anyone explain me and comment the Newtonian viscosity equaton?
I assume you mean, why is the force of friction in a fluid proportional to the velocity gradient in it? (the proportionality factor being the viscosity)
The simplest answer is that meticulous experiments showed that that was, indeed, a very good approximation.
There's a lot more to say about this, of course, but I'll let this be enough for today
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