A pressure-pump has a horizontal tube of cross-sectional area 10 cm...

Volume of water coming out of the pipe per sec $= Av = 10 \times {10}^{- 4} \times 20 = 0 . 02{m}^{3}{s}^{- 1}$

Mass of water hitting the wall per second $m = 0 . 02 \times {10}^{3} = 20kg{s}^{- 1}$

As the water hits the wall normal to the wall and stops, the change of momentum of water hitting the wall is $= m\left(0 - \left(- v\right)\right) = mv$

$= 20 \times 20 = 400kgm{s}^{- 2}$

Since, the change of momentum obtained is per unit time, it is same as the force.

Thus, the force acting on the wall by water = force experienced by water jet$= 400N$.