This website explains the reason why things decrease in velocity when they go on a flat surface. It’s because of the rule of thumb. The rule is that when a object is slowing down, the direction of the acceleration is in the opposite direction object’s motion. The diagram below shows this process. The green arrow indicates the direction of the velocity and the green arrows indicates the direction of the acceleration. When you ride in a car and you come to a urgent stop at a red light you can feel yourself leaning forward. Your motion is forward but you are negatively accelerating, or slowing down.
