N balls per second are thrown on the wall at right angles to it.
mass = m
initial velocity = - u before hitting the wall, towards the wall
final velocity = u after hitting the wall and rebounding, away from the wall
Change of momentum of the ball during collision = m u - (-m u) = 2 m u
As n balls are hitting the wall in 1 sec, the total change of momentum of all the balls together in one second = 2 n m u
Force = change of momentum per second.
Force exerted by the wall on all the balls = 2 n m u
Hence the force experienced by the wall = 2 n m u, as action and reaction forces are equal and opposite in direction.