Why does a helium filled ballon move forward in a car when the car is accelerating?

Question

I noticed that when I had a helium filled, latex ballon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the ballon went from the back of the car to the front of the car. The balloon didn't have a string. This became a game with my 4 year old as we drove home. We figured out where the balloon would go based on how fast I accelerated, turned corners etc. I expected that it would act a lot like the water in a cup does, but it was the total opposite it seemed. What forces caused this behavior? I assumed it has something to do with the fluid dynamics in the closed cabin, but I can't figure it out. Help?

Answer 1

The helium filled balloon is less dense (lighter) than the surrounding air, so it wants to float on top of the air. As your car accelerates, the more dense (heaver) air is pushed to the back of the car, so the balloon tries to "float on top of it" by moving to the front of the car.