Sunday, November 19, 2006

Balloon Puzzle

Here's anther easy puzzle before I throw in a hard one.

You are stopped in a car with a helium balloon floating in the passenger compartment. All the windows are closed and the AC/vents are turned off. The car accelerated forward. With respect to the passenger compartment, does the balloon move forward, move backward, or stay stationary? And why?

3 comments:

Dr. Kim, the Aero guy said...

It moves forward. I have actually asked this question on a quiz in a course I taught at Caltech many years ago.

When you accelerate, the air, which is denser than helium, tends to try to move towards the back of the car. The lighter helium, being bouyant, gets shoved forward by the denser air that's trying to move backwards.

Or you could think of the combination of acceleration and gravity and a tilted gravity, and the balloon floats "upward" in this modified gravity field.

Or you can think about the pressure gradients, ...

michael said...

Dr. Kim is totally correct! woohoo! Not that we would doubt him. He should have his own science TV show, like Bill Ney the Science Guy. We need a name for the show ... Dr. Kim the Science ____. OK, I'll have to work on that one.

michael said...

oops: that's Bill Nye (not Ney) ... sorry Bill.