Say you fill up a balloon with air. Then, without tying the balloon, you let go. The balloon would be propelled up and fly around the room.

This thrust is generated from Newton's third law: with every action, there is an equal and opposite reaction.

However, if you filled up a balloon with water, then dropped it like in the previous scenario, the balloon would just fall down due to the force of gravity. What makes the outcomes different? Theoretically, as the water flows out, shouldn't there be an equal and opposite reaction that propels the balloon upwards as in the previous example?

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    $\begingroup$ Yes there would be, but is it sufficient to counter the increased weight of the balloon? Remember that water is thousand times dense than air. $\endgroup$ – Deep Jul 24 '17 at 5:01
  • $\begingroup$ Just realized how stupid my question was. Thanks for the answer! $\endgroup$ – Frank Jul 24 '17 at 5:02
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    $\begingroup$ @Deep try not to answer in comments, post an answer instead. That's what answers are for, after all. $\endgroup$ – Kyle Oman Jul 24 '17 at 5:07
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    $\begingroup$ @KyleOman You are right. Here I was only giving hints and did not intend to answer the question. $\endgroup$ – Deep Jul 24 '17 at 5:08
  • $\begingroup$ Note that you can buy 'water rockets' as children's toys and they have much higher delta vee than a plain inflated balloon. They are filled with a mixture (about 50/50 by volume) of water and compressed air. $\endgroup$ – dmckee --- ex-moderator kitten Jul 24 '17 at 17:21

The balloon doesn't fly in air for every gas. If you fill the general air in the balloon, it will not fly. The density of filled gas must be lower than the surrounding air. Similar case is in water. The density is same inside and outside the balloon. So upward thrust is zero.

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    $\begingroup$ They are not talking about floating, this answer is not correct. $\endgroup$ – JMac Jul 24 '17 at 10:19

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