Similarly, one can write (or develop) y-momentum balance equation

In vector form

1^st term: Accumulation or transient or unsteady state  
2^nd term: inertial (convective momentum flux)
3^rd term: pressure force
4^th term: viscous forces
5^th term: External body-force
We call this equation Navier-Stokes (NS) equation
Assumptions:  (1)  (incompressible fluid)
                         (2) Newtonian fluid
                         (3) Laminar flow