Air Pressure on this Moment of Science. Here's a neat experiment for you to try the next time you hop in your car.
While you drive, have the passenger hold onto the string of a helium-filled balloon. When you start to slow down, you'll find that the balloon moves backwards. It's just the opposite of what you expect. Here's what's going on. First we have to remember that air has mass; it weighs something.
When the car slows down quickly, you'll feel yourself being pushed against the seat belt. That's because the brakes are stopping the car but not you. You keep moving forward. The same thing happens to the air in the car. When the car stops, the air keeps moving forward. This results in higher air pressure in the front of the passenger compartment. The higher air pressure in front of the balloon pushes it backwards.