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## Underwater Balloon

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 Puzzle ID: #13531 Fun: (2.79) Difficulty: (1.88) Category: Science Submitted By: Bobbrt

You have a rubber balloon with a string attached to it, and a weight attached to the other end of the string. You put just the right amount of air in the balloon such that if you submerge the balloon and weight under water to a depth of 30 feet, the balloon will not rise or sink (its buoyancy force is exactly balanced by gravity).

You then pull the balloon and weight down another 30 feet. If you let the balloon go at this depth, will it rise or sink?
Assume that the water temperature is the same at both depths.

The balloon will sink. At a depth of 60 feet, the water pressure is greater than it is at 30 feet (by about 15 psi). Because of this, the balloon will shrink, causing the balloon/weight system to increase in density (the total volume is smaller while the total mass stays the same). Since the system is now more dense than it was at 30 feet, it will sink.
Also important is the fact that since water is pretty much incompressible, the water density at both depths will be about the same.

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 cccc Jul 15, 2003 Whoa. That was harrrrrrrrrrrrrrrrd! jimbo Nov 28, 2003 Excellent teaser. I first thought that the additional pressure would push it up but then I thought about what would happen to the balloon. BTW keep up your informative explanations on some of the other peoples teasers. They are often much better than the original answers. curtiss82 Dec 24, 2003 This is a good teaser. drussel3 Jun 17, 2005 I am trying to understand this teaser, but I am having difficulty. Sounds a bit in-depth. beegum Dec 13, 2005 Great problem TheEarthling Dec 27, 2005 I got this one! vbguy101 Aug 22, 2006 Thought so!