The momentum equation is based on the law of momentum or momentum principle which states that “the net force acting on a mass of fluid is equal to the change in momentum of flow per unit time in that direction”. Mathematically, force acting on the fluid,

F = d(mv)/dt

where

mv = Momentum.

The impulse – momentum equation is given by

F x dt = d(mv)

It states that ‘the impulse of a force (F) acting on a fluid mass (m) in a short interval of time (dt) is equal to the change of momentum [d(mv)] in the direction of force.