Volume of water coming out of the pipe = A * v
= 20 cm² * 10 m/sec
= 0.02 m³/sec
mass of water hitting the wall = m = 0.02 m³/sec * 1000 kg/m³
= 20 kg/sec
As the water hits the wall normal to the wall and gets rebounded at the same speed, the change of momentum of water hitting the wall is:
= m (v - (-v)) = 2 m v
= 2 * 20 kg /sec * 10 m/sec
= 400 kg-m/sec²
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.
= 400 Newtons.