Question

A hydrogen-filled ballon stops rising after it has attained a certain height in the sky. This happens becausel

Answer 1

A helium filled balloon rises because the weight of air displaced by the balloon is less than its weight i.e. greater up thrust (U).

But, U=vρg where ρ is the density of air.

The density of air decreases with increasing height. When the helium balloon rises to a certain height where density and consequently up thrust is such that it balances the balloon’s weight, the balloon stops rising further up and it halts.

I hope u understood  !!