Zephyr stands inside a train that is moving at a constant velocity and jumps straight up in the air. What happens to Zephyr's vertical position relative to a specific spot on the floor of the train?

Respond very briefly.
Zephyr lands back on the same spot on the train floor from where he jumped. The reason is that when Zephyr jumps, he already has the forward horizontal velocity of the train. Since there are no external horizontal forces acting on him (assuming the air inside the train is moving along with the train and there's no significant air resistance), his horizontal velocity remains constant as per Newton's first law of motion, which states that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Gravity only affects his vertical motion, pulling him back down to the floor after the jump, not affecting his horizontal motion. Therefore, Zephyr's jump does not change his position relative to the train's floor horizontally, and he comes down in the same spot he left from.