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.
