Whenever an object is in projectile motion, that is, it has 2-dimensional motion in the x and y axis, the resultant force on the object is in the y-direction.
This is because once the object has been projected, or the ball has been kicked in this case, there is no longer a force being applied on it in the x-direction. The air resistance is also neglected so the ball's final velocity in the x-direction is equal to its initial velocity in the x-direction.
However, the force of gravity cannot be neglected and causes the ball to come downwards. Therefore, after the ball has been projected, the net force on the ball is downwards, due to gravity.
