Passenger A sits inside a moving train and throws a ball vertically upward. Passenger B, seated beside A, and Observer C, standing on the platform outside, both watch the ball. How does each describe the ball's motion?

1. Passenger B shares the train's reference frame, where the ball only moves up and back down. 2. Observer C is in the platform's reference frame, where the train and ball both carry a horizontal velocity. 3. So C sees the ball trace a path with both vertical and horizontal motion, while B sees only vertical motion. _Source: OpenStax Physics (CC BY 4.0), Ch 2 "Motion in One Dimension", section 2.1 Relative Motion, Distance, and Displacement_