Moment of momentum equation state that "The resulting torque acting on a rotating Fluid is equal to the rate of change of moment of momentum.

let $V_{A} and V_{B}$ are the velocity of nozzle 1 & 2 and $r_{1}$ and $r_{2}$ their respective radius of curvature

Momentum of Fluid at section 1

= $mass \times Velocity= pQ \times Va$

Where Q is the flow rate of liquid

Moment of momentum per second of fluid at section 1 =$(PQV_{A})r_{1}$

Similarly moment of momentum per second of fluid at section 2 =$(PQV_{B})r_{2}$

Rate of change of momentum=$PQV_{B}R_{2}-PQV_{A}r_{1}$

=$PQ(V_{B}r_{2}-V_{A}R_{1})$

According to the moment of momentum principle

Resultant torque=rate of change of motion in momentum Torque T=$PQ(V_{B}r_{2}-V_{A}r_{1})$

this equation is known as moment of momentum equation