From: Ben Pittenger (ben_pittenger@n2mail.com)
Date: Tue May 22 2001 - 07:49:25 PDT
Date: 22 May 2001 14:49:25 -0000 Message-ID: <20010522144925.7404.qmail@ninelives.chek.com> From: "Ben Pittenger" <ben_pittenger@n2mail.com> Subject: Re:Helium balloons in the car
I have never observed the phenomenon of a helium balloon appearing to move 'forward' in a car as the car accelerates, but it sounds like a fun demonstration. As a mental experiment, it makes sense. But I'll let all of you decide whether it makes as much sense as I think it does.
I envision a glass of water on the dash of a car. As the car accelerates forward, it pulls the glass with it because the glass is rigid and is in direct contact with the dash, and because the friction between the glass and dash is strong enough to pull the glass along. (with a fast enough acceleration, the static friction would not be sufficient, and the glass would remain relatively still compared to the car and would appear to slide backwards off the dash) However, the water in the glass will not accelerate as quickly as the car or the glass because it is not as tightly bound to the car, and it is not as rigid as the glass. So the water would be pushed forward by the glass, but because it is a fluid, it will deform and appear to well up on the back side of the glass as the car and glass accelerate out from under it (sort of like a bug deforming on a windshield). But at the same time any air in the glass, being a less dense fluid, would be displaced by the water and would seem to move forward to the fro
nt of the glass.
Now for the helium balloon: Just as the water in the glass would tend to stay still as the car accelerates forward, the air in the car would 'tend' to stay still until forced to move. The most dense gases would resist acceleration the most and would well up, or collect, at the back of the passenger space. Any less dense gas (such as a helium balloon) in the passenger space would be displaced by the more dense air, and would feel a forward force relative to the car as the more dense gas piled up behind it. Therefore the balloon would appear to move forward relative to the interior of the car, and relative to any passengers within the car.
I guess we could use Newton's first and second laws of motion to explain the gas tending to stay still and the more dense gas accelerating more slowly than the less dense helium, respectively, but that would be a bit too academic.
So that's my shot. Go get it, sharks!
Ben Pittenger
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