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.