A stream of a liquid of density flowing horizontally with speed v...

Cross-sectional area $A=\pi r^2$, Volume of liquid flowing per second $=A V=\pi r^2 V$ Mass of the liquid flowing out per second $=\pi r^2 v \rho$ Initial momentum of liquid per second = mass of liquid flowing $\times$ Speed of liquid $=\pi r^2 v \rho \times v=\pi r^2 v^2 \rho$ Since the liquid does not rebound after impact, the momentum after impact is zero. $\therefore$ Rate of change of momentum $=\pi r^2 v^2 \rho$ According to Newton's second law, the force exerted on wall = rate of change of momentum $=\pi r^2 \rho v^2$