Science Puzzle
Which Way Does the Balloon Lean When the Car Brakes?
You are in a car holding a helium balloon by its string. The car brakes hard. Your body lurches forward toward the windscreen, as everyone expects.
Which way does the helium balloon tip?
The Answer
The balloon tips backward, toward the rear of the car, the opposite way to you. This looks bizarre until you think about the air.
When the car brakes, everything with mass wants to keep moving forward, including all the air in the cabin. The heavy air surges toward the front and packs together there, leaving the back of the cabin slightly less dense. A helium balloon always moves toward the lightest air, the same reason it floats up against gravity. So it drifts to where the air is now thinnest: backward.
Your body, being denser than air, does the ordinary thing and lurches forward. The balloon, being lighter than the air around it, does the exact opposite. It is a floating object in a fluid, and the fluid decided which way is "up".
The principle: Buoyancy under acceleration. Braking makes dense air pile toward the front, so the less dense helium balloon is pushed the other way, just as it rises against gravity in still air.