Braingle: 'Underwater Balloon' Brain Teaser

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.

Hint

The overall density of the water/weight system at 30 feet is exactly the same as the density of water. How does the density of the system change at 60 feet?
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Comments

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!