Final answer:
The scenario given in the question is a classic example of relative motion in physics. It illustrates how different observers perceive the motion of a ball differently due to their own motion relative to the ball.
Step-by-step explanation:
The question you've asked deals with relative motion and how different observers perceive the movement of objects depending on their own frame of reference. Specifically, considering a passenger inside a train who throws a ball vertically upwards: Passenger B (another person inside the train) would see the ball going up and then coming down in a straight line, with no horizontal motion, because Passenger B shares the same horizontal velocity as the train and the ball.
However, Observer C, who is standing on the platform outside the train, would see the ball arcing through the air because the ball retains the horizontal motion of the train in addition to the vertical motion given by the throw. This means the ball travels in a parabolic trajectory relative to Observer C's point of view, exhibiting both vertical and horizontal motion.