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Physics/helium balloons


Why do helium-filled balloons, like you'd find at a birthday party, lose their ability to stay up, after  a day and then wind up on the floor after the third or fourth day?

Hello charlesfield,

They loose their buoyancy because their helium slowly leaks out. Helium is a small atom and the balloon has small pores. When you find it on the floor, the skin of the balloon is probably noticeably limp compared to when it was freshly filled.

When the balloon is filled, it is under slight pressure and that increases each helium atom's probability of finding its way out. A balloon filled with plain air does not loose its contents as quickly because most of those molecules are significantly larger.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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