1. The problem statement, all variables and given/known data 2. Relevant equations Navier-Stokes equations of motion 3. The attempt at a solution I did everything well but, my question is, why we assume last term rho*gz=0? in the N-S equation? Also why do we use Navier Stokes equations in terms of velocity gradients for newtonian fluid with constants rho and mu, and not Navier Stokes in terms of Tau? When to use Navier Stokes in terms of Tau, and when to use Navier Stokes in terms of Rho and Mu?