The boy’s foot causes the motion. His foot is the one that causes the ball to roll down the hill.
I believe the answer is "When a neutral atom looses an electron to another neutral atom, two charged atoms are created."
Force is defined as the rate of change of momentum.
The initial amount of momentum is

because water stops when it hit the wall total change of momentum must be

.
Now let's calculate the force.

We need to find

. This is the amount of water hiting the wall per second.

Our final formula would be:

And now we can calculate the answer: