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.

(In reply to Solution by Bob)

Hunh.  I didn't think of that.

My initial thoughts were simply that the balloon moves backwards in the car when you accelerate (meaning your forward speed increases), it would move to the left when you turned right, and move to the right when you turned left.  I was purely thinking about the inertia of the balloon.  I was also paralelling it with my own movements in a car (or if in the back of the car was a perfectly flat and level board with a ball on it... where would the ball roll).

But when you reminded us that the balloon is less dense than air, your solution makes so much sense.  The AIR molecules in the car are affected by inertia as well.  As the car accelerates, the air molecules move backwards, packing into the back of the car a bit and leaving the front less dense, thus created the pressure difference you mentioned.  Same goes for the turns.  The air is moving the way our body would move, creating a pressure difference and pushing the balloon into the turn, as you said.

Granted, the air does this when we (or a ball on a board) are in the car too, but you pointed out that the key is that the ball and I are denser than the air, while the balloon is not.  So the inertia of the ball or myself "overpowers" the air's pressure difference, but for the balloon the pressure difference is more significant than it's own inertia.

Cool!  You rock!

Afterthoughts:  So wait, does this mean that if someone put me in a water tank (completely full, no sloshing), tethered me to the bottom, and then drove me around, that I would behave like the balloon?  I'd float forward when the accelerated, and float into the turn?  Freaky!  Of course, I'd be relying on holding my breath because if I added a scuba tank to myself, I don't think I'd be less dense than water still.

Or am I not less dense enough to have this effect?  Do you have to be less dense by some factor to make pressure over power inertia?  Also, water is not as compressible as air, so the pressure difference might not be big enough.  Still cool to think about.

What if it weren't completely full, there was some sloshing room?  Still same effect or probably not?  This is fun!

Edited on December 16, 2004, 3:10 pm

Posted by nikki on 2004-12-16 14:38:10