If you took a room sucked the air completely out and put in a balloon filled with helium, would the balloon sink to the ground?
The baloon would explode in the room’s vacuum.
Hit ‘submit’ too quickly there…
After the balloon pops, the helium will disperse evenly throughout the room and the broken balloon will sink to the floor.
So yes, the balloon will sink, just not in the manner you might have anticipated.
Are you sure the balloon would pop? It’s not as if a complete vacuum, pulling evenly across the balloon, is going to add that much stress. Depending upon how tough your balloon is, I think it would stay intact (especially a mylar balloon – they’re pretty tough).
In any case, the balloon would sink. A balloon rises only because gravity induces a pressure differential in the air that causes a net upward lift (as we’ve discussed in many a thread before this). If there’s no air in the room, there’s nothing to create a pressure differential in, and nothing to push the baloon up.
Larry Niven had a lot of fun with this (because he was doing a riff on Edgar Rice Burroughs’ Martian ships) in the novel Rainbow Mars. But that was acknowledged fantasy.
If the balloon were to remain intact then it would fall to the floor like a rock, not the gentle floating that you would usually associate with a balloon. This is because there would be no air to buoy the balloon up or resist its decent. In fact in a vacuum a balloon and a cinder block would fall at exactly the same rate regardless of what the balloon was filled with.
It would pop.
Each square inch of the balloon is pushing out with a force equivalent to 15lbs. A 5 inch square of balloon rubber is pushing outwards with x^2 pressure, 375lbs. If you stretch a piece of balloon rubber on a 5" ring, can a 400lb man pop the rubber by pushing down on it with his fist? Sure.
On the other hand if your balloon was vastly underinflated (perhaps just enough so it would barely rise upwards in air) then it would greatly expand when you placed it in a vacuum, and the internal pressure would drop.
On the original question, imagine an air-filled plastic bag under water. It lifts upwards with a very large force. Remove the water and the bag no longer lifts. Same with helium in air: remove the “sea” of heavy air and then you’ll discover the actual weight of the helium+rubber. Helium only goes upwards because it is immersed under a heavy “sea” of dense transparent gas.
It wouldn’t pop, all you have to do is put enough helium in for it to hold its shape, it would take very little in a vacuum.