Part of the show Catalysts for Cleaner Environments and Future Energy
Graham in London asked:
Take a helium balloon and hold it on a piece of string inside a car. Watch what happens to the balloon when the car breaks - it moves backwards. So it goes the opposite way to other objects in the car. What's happening here?
When you do an emergency stop, the car is accelerating everything in it backwards. This means that everything relative to the car rushes forwards. The balloon is obviously floating in the air. But if it's a helium balloon, the air is heavier than the balloon itself. This means that the air wants to rush forwards more than the helium balloon is going to want to rush forward. The air will then push the helium balloon out of the way, making it move backwards.