If a tiny helicopter was hovering inside a moving car, would it move with the car?

[Chronos]

I don’t think it’s so much pressure difference that moves the ballon, but acceleration, which is the same as gravity, causes the air to ‘sink’ toward the back window and the ballon to ‘float’ toward the front.

Six of one, half-dozen of the other. The air “sinking” forward causes the pressure difference, and the effect of the pressure difference is what you called “floating”.

And if I had a dime for every time a physics student thought of momentum as a force, it’d probably double my stipend.