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Helium Balloons

Tie a helium-filled balloon to the floor of your car so that it is suspended half-way to the roof. Close the windows, turn the car on and accelerate. What happens to the balloon?

1. Stays where it is
2. Moves forward
3. Moves backward

Got it?

The answer is that the balloon moves forward, which is easy to verify experimentally. Why is this so? The standard explanation is that the car’s acceleration causes the air in the car to get ‘pushed’ backwards (in the car’s frame of reference), creating higher air pressure in the rear of the car than in the front. This pressure difference pushes the balloon forward.

Are you convinced by this explanation? I’m not. Not because it’s wrong, but because I keep thinking: But if I took the air out of the car, the balloon would go backwards. So whose to say this effect isn’t greater than the air-pressure effect?

So, here’s my take on this. When you release a helium balloon, it goes up. That’s a familiar phenomenon caused by gravity: Gravity pulls air downwards, which means air pressure increases as you go down. This pressure difference, combined with the fact that helium is less dense than air, causes the balloon to rise.

Let’s return to the car. The car is an accelerating system. According to the equivalence principle, an accelerating system is the same as a system in a gravity field. So the balloon-in-the-car system is the same as the balloon-in-your-hand system. Hence, the balloon in the car will be behave just like the balloon in your hand: It will go ‘up’, which in the car system means ‘forward’.