Assertion :A helium filled balloon does not rise indefinately in air but halts after a certain height. Reason: Viscosity opposes the motion of balloon

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Assertion is incorrect but Reason is correct

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Solution

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
The balloon initially rises in air because the weight of the displaced air i.e. upthrust is greater than the weight of the helium and the balloon. Since the density of air decreases with height therefore the balloon halts at a particular. height where the density of air is such that the weight of air displaced is just equal to the weight of helium gas and the balloon. Hence the net force acting on the balloon is zero and the balloon stops rising.

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