text: Your're seated in the car, and so your body is directly effected by the motion of the car, and thus moves forward when you apply the breaks.\n\nHowever, the helium balloon is simply floating in the air, and it's actions are not affected by the car, but by the air around it. When you brake the car, the balloon doesn't actually move backwards, it pretty much remains stationary (due to the air pressure moving in front of it), and the car moves forward (giving it the balloon the illusion of moving backwards)\n\nWhen the car moves forward, that same air pressure, keeps the balloon in place and helps it move forward. (You may find that if you suddenly press the accelerator and shoot forward, that while your head is thrown against the seat rest, the balloon will actually move forward.)