Physics
Explain the following:
A balloon filled with hydrogen rises to a certain height and then stops rising further.
Answer
A balloon filled with hydrogen rises to a certain height and then stops rising further, because the density of air decreases with altitude. Therefore, as the balloon gradually goes up, the weight of the displaced air (i.e., upthrust) decreases. It keeps on rising as long as the upthrust on it exceeds it's weight. When upthrust becomes equal to its weight, it stops rising further.
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