A passenger train moves east at 15 m/s relative to Earth. Inside the last car of the train, a passenger walks toward the rear of the train at 1.5 m/s relative to the train. A tennis ball rests motionless on the aisle floor of that same car.
Two additional observers are present:
• Observer E stands on a nearby station platform.
• Observer T rides on a second train that happens to move east at 15 m/s (the same speed and direction as the first train) while adjacent to it.
(a) Identify which of the three observers (the passenger, Observer E, or Observer T) sees the tennis ball at rest. Justify your answer qualitatively.
(b) For each observer, describe both the direction (east or west) and the relative speed of the walking passenger. Rank the three measured speeds from greatest to least and explain your reasoning.
(c) The passenger lightly taps the tennis ball so that it momentarily begins to slide but soon comes to rest again relative to the train car. Explain why, despite the different measurements of motion in parts (a) and (b), every observer would agree that the horizontal acceleration of the ball is essentially zero once it has come to rest relative to the train car.