Underwater Balloon
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.
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