NOVA Online | Voyage of Doom | Balloon-in-Car Puzzler 2

Toward the Front
Puzzler 2

Absolutely right! When the driver hits the gas, you're thrown back into your seat. The balloon, on the other hand, moves forward! When the driver hits the brakes, your body lurches forward. And what happens to the balloon? It flies toward the back of the car! Maybe you've noticed this for yourself. Maybe you haven't and you don't believe it. Just watch a helium-filled balloon the next time you have one in your car.

Now here comes the interesting part of this puzzler: the explanation about why this happens.

Here's a hint that you may have already picked up on: The answer has to do with buoyancy (otherwise, this question wouldn't have been included in this Hot Science).

A helium-filled balloon floats because the surrounding air (which is really a fluid) is being pulled down with a greater force. It's as though the air is pushing itself down, trying to get to the ground, and the lighter balloon is in its way. The balloon is, in effect, being pushed upward by the heavier air. (A cork being held underwater wants to rise for the same reason.)

In the car, you're not the only thing that gets pushed back while accelerating. A toy on the seat feels the force, as does the seat itself, as does the fuzzy dice hanging from the rearview mirror. Even the air feels the force—and moves toward the back of the car because of it. Sure, the balloon also feels the force and, like the air, has the freedom to move back, but the denser air pushes its way past. And with the heavier air crowding the back of the car, there's no place for the balloon to go but forward.

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