818. Why doesn't a helium balloon pop when it reaches the ceiling?
The buoyant force lifts the helium balloon upward—the denser air flows downward to fill the space vacated as the balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon's skin is gentle enough and spread out enough that it doesn't injure the rubber. The balloon simply comes to a stop and remains suspended until enough helium diffuses out of the balloon to cause it to descend.