Zephyr is holding a helium balloon on a string while standing inside a train that is moving at a constant velocity. If the train suddenly stops, what happens to the balloon?

Respond very briefly.
When the train suddenly stops, the air inside the train momentarily continues moving forward due to inertia. Zephyr, and any loose objects not secured to the train, are also pushed forward. However, the helium balloon will move in the opposite direction (toward the back of the train). This is because the helium inside the balloon is less dense than the air around it. When the train stops and the air is pushed forward, it creates higher pressure at the front and lower pressure at the back. The balloon is carried toward the lower pressure area at the back of the train. This is an example of how air pressure differentials affect lighter-than-air objects, even in the absence of direct air currents.