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

Answer 1

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.

Answer 2

This is an example of something called "the Equivalence Principle" in General Relativity. This principle, first described by Albert Einstein in 1907, states that it is impossible to tell the difference between the force of gravity in one direction and an acceleration of a reference frame in the opposite direction. In other words, to all the people and objects inside a car accelerating FORWARDS, it acts, feels, and looks exactly like there is a gravitational force BACKWARDS (towards the back of the car). This is why heavy objects (like the driver, passengers, and objects left in the back seat or on the dashboard) move towards the back of the car as it accelerates - they 'fall' in the direction of this 'pseudo-gravity' that they experience. This is also why the helium-filled balloon moves to the front of the car - it "floats" in the direction opposite the 'pseudo-gravity', just like it floats when it is released outside the car into the air.

Try this experiment at home (with someone ELSE driving!). Hold a helium-filled balloon by a ribbon or string, inside a car, and have the driver take a sharp left turn. In order to make that sharp left turn, the car must accelerate to the LEFT. By the Equivalence Principle, everything inside the car should act as if it is experiencing a force of gravity to the RIGHT. Heavy objects (you, for instance), will feel a force pushing you to the right - you will 'fall' towards the right side of the car. Because it is less dense than air, the helium-filled balloon will 'float' in this same gravity, towards the LEFT. What do you predict will happen to the helium-filled balloon when the driver makes a sudden, sharp turn to the RIGHT? What will happen to the balloon when the driver slams on the brakes?

Answer 3

Balloon moves forward due to the difference in the density of air inside and outside of the balloon.