Consider a jet of water coming out from the nozzle, strike a flat vertical plate as shown in the figure

Let,

v = velocity of jet,

d = diameter of the jet,

a = area of the cross-section of a jet

$a=\dfrac\pi4d^2$

The jet after striking the plate will move along the plate, since the plate is at a right angle to the jet, the jet after striking will get deflected through 90$^\circ$

So, the component of velocity of the object, in the direction of jet, will be zero after striking.

$F_n=\dfrac{\text{Initial Momentum - Final Momentum}}{\text{Time}}$

$F_n=\dfrac{\text{(Mass X Initial Velocity - Mass X Final Velocity)}}{\text{Time}}$

$F_n=\dfrac{\text{Mass}}{\text{Time}}[\text{Inital Velocity - Final velocity}]$

$F_n = \text{(Mass/sec)} \times \text{(Velocity of jet before striking - Velocity of jet after striking)}$

$F_n=\rho av [v-0]$

$F_n=\rho av^2$....................(1)