Question

Explain why a balloon filled with hydrogen rises up to a certain height and then stops rising.

Answer 1

1. For a balloon to rise in the air, the buoyant force acting on it in the upward direction must be greater than the weight of the balloon.
2. Filling the balloon with hydrogen makes the weight of the balloon less than the buoyant force acting on it and hence the balloon rises.
3. Buoyant force acting on the balloon is directly proportional to the density of air.
4. As the balloon goes on rising, the density of air keeps on decreasing because the density of air decreases with altitude.
5. Buoyant force acting on the balloon also keeps on decreasing with altitude.
6. At some point in the atmosphere, the buoyant force becomes equal to the weight of the balloon and hence the balloon stops rising.