Volume of water coming out of the pipe per sec =Av=10×10−4×20=0.02m3s−1
Mass of water hitting the wall per second m=0.02×103=20kgs−1
As the water hits the wall normal to the wall and stops, the change of momentum of water hitting the wall is =m(0−(−v))=mv
=20×20=400kgms−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.