Which way will a helium balloon move if you slam on the brakes of your car?

If you slam on the brakes of your car, a helium-filled balloon will move towards the BACK of the car.

Since helium is lighter than air, a helium-filled balloon will float in air. This is an example of Archimedes Principle. This principle was first described by Archimedes, in the third century BC - the balloon experiences a buoyant force equal to the density of air times the volume of the balloon times the acceleration due to gravity, or

FB = p V g,

This buoyant force is directed OPPOSITE the direction of gravity (UP, whenever gravity is pointed DOWN).

Fast-forward 23 centuries, to 1907, when a young patent clerk named Albert Einstein first described something called the Equivalence Principle. This principle, the foundation of the Theory of General Relativity, states that it is impossible to distinguish between an accelerated reference frame and a gravitation-like force. In English, that means that a car decelerating (like when you slam on the breaks) feels and looks to everyone and everything inside the car as if there is a force of gravity towards the FRONT of the car. Heavy objects (like the driver, the passengers, and any loose objects sitting on the dashboard) act as if they are 'falling' forwards ('falling' in the direction of this 'gravity'). The helium-filled balloon will move towards the BACK of the car, because it will 'float' in the direction OPPOSITE this gravity.

For more detail, visit the Wikipedia or WikiAnswers pages on the Equivalence Principle, Archimedes Principle, and General Relativity.