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Originally posted by UofCinLA
Let's assume you built a a pulley system and hooked a bag to it. You will have friction losses in towing the bag down (pulley, water drag and weight of the line that is on the way up). Assume you expend no energy to fill the bag and let it go up. You will have even larger frictional losses on the way up because you now have a big inflated bag AND you are fighting gravity.
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Still doubting - the potential energy difference is from sea level to say 300 feet below sea level. But you are adding a gas at 300 below which while lighter than water still has a mass. The kinetic energy to go up will now be lower than the potential since you have added mass to the system.... No free lunch and Newton wins again....