Lerato is standing on a train platform with a helium-filled balloon in her hand. The balloon is tied to a string that Lerato is holding tightly. As a train swiftly passes by the platform without stopping, what happens to the balloon?

Respond very briefly.
As the train passes by, the balloon will tilt toward the direction the train is traveling. When the train displaces the air around it, it creates a wake of lower pressure behind it. The higher pressure air on the opposite side of the train will push the balloon into the lower pressure area in the wake of the train. Although the string Lerato holds maintains the balloon's proximity to her, the balloon's lighter-than-air property allows it to move more freely in the air currents, hence the tilt in the direction of the passing train.