Final answer:
The direction of a ball's momentum is changed by an external force when it collides with a wall, conforming with Newton's third law of motion. The wall exerts a force in the -x direction on the ball, reversing the component of its momentum in that direction.
Step-by-step explanation:
When a ball collides with a wall, the direction of the ball's momentum is indeed changed by an external force. The wall exerts a force on the ball in the direction opposite to its initial motion, commonly referred to as the -x direction, while the ball exerts a force on the wall in the +x direction. This is consistent with Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. Therefore, the change in momentum of the ball is in the -x direction.
According to Newton's second law, the change in momentum is equal to the impulse imparted to the ball. If we let u be the speed of the ball before and after collision and m be the mass of the ball, and we consider the collision specifically on the x-axis (perpendicular to the wall), the change in momentum can be calculated accordingly. As such, option A is correct: Yes, the direction of the ball's momentum was changed by an external force.