You work for a balloon delivery service and you are delivering a single, helium-filled balloon in your car. To prevent the balloon from bouncing around on the ceiling while you are driving you have tied a string with a weight on it to the balloon. The weight is resting on the floor and the balloon is floating just below the ceiling.

When you accelerate, does the balloon stay where it is, move backward, or move forward? What does it do when you make a turn?

Assume all the windows are closed and the vents are turned off so there is no air flow inside the car to affect the balloon.

Helium is less dense than air, and according to Archimedes' Principle it will move to the direction of the lowest air pressure. Thus, when you accelerate, a temporary disturbance in air pressure is created, with higher pressure being at the back of the car and lower pressure in the front. On acceleration the balloon will float forward. At constant speed, the balloon will not move. Similarly, when you turn, the balloon will float into the turn (i.e., move left when you turn left).

I'm assuming there is air in the car, since without air there can be no air pressure. Also, if it were a vacuum, the driver would slowly lose consciousness and die.

Posted by Eric on 2004-12-16 13:40:40