In fact, in all these cases I consider either we observe the boundary condition, the pressure term or sometimes we neglect that because we are considering (( )). Basically, it is observed in a pressure term and if this is 0 that, these two things is 0 that means pressure is independent of y and z that means p can be written as a function of p x only. And in the process, what will happen to the x component of the equation of motion? This is from the y component of the equation of motion and this comes from the z component of the equation of motion and then equation of motion the x component will give up del u by del t because inertia terms are negligible, this is equal to minus rho into del u by del t is del p by del x minus del p by del x, the body (( )) is a involved with this plus into be mu into del square u by del y square plus del square u by del z square because u is a function of the new del square u that will show u is a function of y z.