1. The problem statement, all variables and given/known data Why do objects that have no external net force acting on them accelerate? Ex. If a ball is on an accelerating train, it will accelerate opposite the direction of the train's acceleration, assuming there is nothing blocking its path of motion and it is not strapped down. My teacher says that we use the fictitious force to explain this motion because there is no net-force on the ball, but is this the actual reason? 2. Relevant equations - 3. The attempt at a solution Does the ball accelerate in my given example because it wants to remain in the same position it was in before the train began accelerating. That's what I think, but this explanation doesn't make sense, because it uses inertia to explain the ball's motion, even though the motion goes against what Newton's First Law says anyways, because the ball is moving even though there;s no net force acting on it.