It is derived moment of momentum principle, which states that the resulting torque acting on a rotating fluid is equal to the rate of change of moment of momentum.

Let, $v_1$ = velocity of fluid at section (1),

$r_1$ = radius of curvature at section (1),

Q = rate of flow of fluid,

$\rho$ = density of fluid.

and $v_2$ and $r_2$ = Velocity and radius at section (2)

momentum of fluid at section 1

= Mass x velocity

$= \rho_Q \times v_1 / s$

$\therefore$ moment of momentum per second at section (1),

$= \rho_Q \times v_1 \times r_1$

Similarly moment of momentum per second at section (2),

$= \rho Q \times v_2 \times r_2$

$\therefore$ Rate of change of moment of momentum

$\rho Q v_2 r_2 - P_Q v_1 r_1$

$= \rho_Q [ v_2 r_2 - v_1 r_1]$

According to moment of momentum

Resultant torque = rate of change of moment of momentum

$T = \rho Q [v_2 r_2 - v_1 r_1]$

This equation is known as moment of momentum equation.