Q 11-04-045NEETJEE MainMedium
A horizontal jet of water with cross-sectional area $2\ \text{cm}^2$ and speed $20$ m/s strikes a vertical wall and falls dead (does not rebound). The force on the wall is (density of water $= 1000\ \text{kg/m}^3$)
Answer: (B) $80$ N
Mass hitting per second: $\rho Av$. Each unit of mass loses momentum $v$:
$$F = \rho Av^2 = 1000 \times 2 \times 10^{-4} \times 400 = 80\ \text{N}$$
If the water rebounded with the same speed, the force would double to $160$ N.
Solution by Sreeraj P, M.Sc Physics