Answer:
Because the path of the ball is perpendicular to the gravitational force.
Explanation:
In the first case, trajectory of the ball has a component parallel to gravity. Therefore, gravity speeds up the ball.
In the second case, trajectory of the ball has a component anti-parallel to gravity. Therefore, gravity slows down the ball.
When ball rolls on the horizontal surface, the trajectory of the ball is perpendicular to the gravitational force. Therefore, gravity doesn’t play any role.
