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.

Very interesting. The balloon must react to acceleration the same way it reacts to gravity.

And what about the ballon in a falling car ?

In the car, I don't feel any acceleration, as I am falling and accelerated freely by gravity. So, the balloon must also be completely free and float around with no relative movement up or down, so that I cannot use it to know that I am falling. I can put it anywhere in the car and it will stay there.

Even if you learn it during several years at school, it is not always easy to remember that gravity is acceleration.

Posted by François on 2005-01-06 18:04:00