Final answer:
When the keeper on team A slides and makes contact with the ball inside the arc, his momentum carries him outside the arc with the ball due to the law of conservation of momentum.
Step-by-step explanation:
When the keeper on team A begins his slide and makes contact with the ball inside the arc, his momentum carries him outside the arc with the ball. This is because when the keeper applies a force to the ball, the ball exerts an equal and opposite force on the keeper, causing him to move in the direction opposite to that of the ball's motion. This can be explained by the law of conservation of momentum, which states that the total momentum of an isolated system remains constant if no external forces act on it.