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Have you tried it? I have. It's quite startling the first time you put a bunch of helium balloons in your car, accelerate away from a stoplight, and have them all crowd up against the windshield.

In an accelerating car, the air sloshes toward the back, producing a pressure gradient. The helium balloons, being ligher than air, will be crowded out by the sloshing air and will drift toward the area of lowest density, which is in the front.

Actually, the WSJ is correct. Helium has a lower density than air, while a head has a higher density. Hence, they will move in opposite directions.

I can assure you that the wsj is correct! I've been transporting balloons a bit lately* and having fun watching them 'lean in' to turns, 'leaning back' while braking, and 'leaning forward' while accelerating.

*transporting balloons correlates strongly with transporting kids

Good grief, I stand corrected.