Once you hear the explanation, courtesy of the host in the YouTube series Smarter Every Single Day, the counterintuitive behavior with this balloon getting around in a minivan under acceleration and deceleration won’t seem so unreal. But until then, it seems that the helium balloon is defying physics and good sense.
Watch the video first, prior to deciding to read the explanation.
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If you listened closely, you’ll find that it’s actually fluid dynamics that explain the physics behind the balloon’s unexpected behavior. Remember, the van is full of air, a blend of gasses composed of many molecules. When the van accelerates forward, the individual molecules resist the motion from the van (inertia) and collect at the back of the van. They displace the helium balloon, which moves forward, as more molecules fill the back. The impact is exaggerated by the fact the the helium is less dense then air, which makes the balloon’s movement even more extreme.