physics and intuition

The balloon continues forward until it bouces off of the windshield. While it has relatively little mass, what mass it has, has nothing to do with it's tendancy to rise in the Earth's atmosphere.

 

It hits the rear window if the leash is long enough.. Of course with the assumption that it doesn't stuck on the ceiling on the way..

 

I can imagine that air (which has more mass per its volume) gets ahead of the balloon (which has less mass than air per its volume). This would cause the balloon to bounce off the air mass crowding ahead of it, and it would go backwards, as TD said. So I was wrong, but now I see.
I also understand now why TD has more points than me.

 

Correct

 

Why? The acceleration relative to the car (which is the opposite of car's decceleration relative to the ground) is the same for both of them, so neither of them gets ahead of the other... The baloon will simply hit the windshield.

 

Maybe, but more likely it will remain motionless relative to the car. I think for the sake of visual aid I imagined the balloon in a large box, so I could be unencumbered in my thinking by intermediate obstacles.
So in addition to simplifying the process, one of my favorite mental imagery tricks is to take the experiment all the way to infinity on one side and to zero on the other.
I learned that from a riddle long ago, one that I will be glad to share, if anyone should ask. But I’ve given you the answer already haven’t I?
So, back to the balloon.
Imagine that instead of air there is water. Well since water doesn’t compress there would be little movement and a compressible ball of lighter material hanging on a tether would most likely just compress, and the flow generated by the fluid behind it (as it moves to compress the balloon) would then create flow around the balloon. If the balloon shell was uncompressible there would be not movement because there would not be a pressure differential based on the non compressible nature of water.
Obviously air compresses and so does the balloon. However at the speeds we are talking about not much compression occurs, given the small masses involved.
So take this experiment to infinity (or something large enough where you don’t have to imagine the minutia) and say the deceleration is going to cause a 50% shift in the distribution of air inside. Trust me on this, you would not want to be in that box at that point.
What is the air inside going to look like?
Well first it’s going to create (once again I visualized this in a completely uncluttered and I guess spherical container) a variable density atmosphere with a relatively large amount of turbulence, even without interior clutter so that means that imagining this with a spherical interior surface makes it harder to visualize, too many variables to consider, remember we are attempting to learn how to simplify and extrapolate to infinity.
So let’s go with a cube instead. Imagine now all of the air being forces towards the front of the cube. Now we know the balloon is tethered, so that would indicate that it is still less dense than the air and therefore has less kinetic energy.
What do you see happening now?

 

Required reading for this course:
http://en.wikipedia.org/wiki/Sokal_affair

After that we can all have a jolly good laugh at how silly some people are.
Leo.

 

Are you insulting me? I'm here for the free coffee, if i can provide entertainment in the process, all the the better.

 

http://www.physlink.com/education/askexperts/ae554.cfm

 

The inertia of the balloon makes it go back initially. The air friction will push it forward until its velocity matches that of the car. It may hit the back window, but there isn't enough data to calculate that.

 

Damn! It does make sense... I need to buy a baloon, now!

 

I love this question because the answer is so counter-intuitive.

I think it ties nicely to Dave's original post.

Too bad he never got a chance to answer it.

Try it next time you have a balloon full of He.

The corners are fun too. It'll blow the kids away!

-Tom

P.S. A telephone booth has about six pounds (3 kg) of air in it.

 

But how fast you got to drive with A a pick up or B a school bus to make it happen

 

http://www.youtube.com/watch?v=plbI-QxdFzk

 

Not that fast.

-Tom