[joexner]

The balloon would probably move toward the front when accelerating and toward the back when the bus slows, if it moved at all.

It helps (me) to imagine an air bubble in a sealed, nearly-full fish tank on that same accelerating bus. The heavier water gets "flung" harder away from the direction of acceleration, and the bubble gets pushed out of the way in the opposite direction. Same principle.

[tzs]

Yup. The movement is quite noticeable. Here's someone trying it [1].

[1] https://www.youtube.com/watch?v=XXpURFYgR2E