This principle is based on the law of conservation of momentum which states that the net force acting on a fluid mass is equal to the change in momentum of flow per unit time in that direction.

By Newton's second law of motion we have

F=ma (But a=$\frac{dv}{df}$)

F=m$\frac{dv}{dt}$

F=$\frac{d}{dt}$(mv)

(m is constant) Equation is known as momentum equation or momentum principle

Also Fdt=d(mv)

In equation (m,v) represents the momentum and Fdt represents the impulse

This is known as impulse momentum principle

Impluse-momentum principle states that "The Force 'F' acting on fluid of mass 'm' in a short interval of time 'dt' is equal to the change in momentum d(mx) in the direction of Force