Zephyr and Keiko are standing inside a train that is moving at a constant velocity. Zephyr is holding a helium-filled balloon on a string. Suddenly, the train begins to brake quickly but smoothly. From Keiko's perspective, who is watching the balloon, in what direction does the balloon move as the train brakes, and why? Respond very briefly.
When the train brakes, Zephyr and Keiko would feel a force pushing them forward due to their inertia. However, the helium balloon will move in the opposite direction, toward the back of the train. This happens because the air inside the train is denser than the helium in the balloon and will be affected more by inertia. As the train slows down, the denser air moves forward and pools at the front of the carriage. The helium balloon, therefore, gets pushed backward by the forward-moving air. To Keiko, it will appear that as the train brakes, the balloon shifts towards the back of the train.