Conservation of momentum: this is Newton's Second Law and it states that Force = rate of change of momentum. Momentum is mass times velocity. The momentum equations are the equations that make it hard to solve problems in fluid dynamics. There are a number of different versions that include a number of different effects. The Navier-Stokes equations are momentum equations, and the Euler equations are the Navier-Stokes equations but with viscosity not included. There is one momentum equation in a 1D problem and three, one in each space direction, in 3D.