Strange helium balloon

When a car stops suddenly, the air inside the car continues to move forward a little longer than the car itself (same as the driver and any passangers in the car), due to the tendancy of an object to maintain its motion until a force acts on it (inertia). The air is stopped by the front of the car's interior and tends to pile up there. As a result, the air in the front of the car becomes slightly more dense than the air in the rear of the car, creating more pressure on the front side of the balloon. This causes the balloon to move toward the rear of the car, where the air is less dense, just as the same balloon released outside will move upward, toward the less dense air.

As Einstein's General Theory of Relativity emphasizes, acceleration due to a change in velocity and the acceleration due to gravity are equivalent. A sudden stop results in an acceleration towards the front of the vehicle,indistinguishablee to its occupants from a gravitational force that would be felt if the car were to be held stationary in a down hill orientation.