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

A hydrogen-filled balloon stops rising after it has attained a certain height in the sky due to the balance between the buoyant force and the force of gravity.

Here are the steps to understand this:

1. When a balloon is filled with hydrogen, it becomes lighter than the air around it. This is because hydrogen is less dense than air.

2. As a result, the balloon experiences a buoyant force which pushes it upwards. This force is due to the difference in pressure between the bottom and the top of the balloon.

3. As the balloon rises, the air pressure decreases. This means the difference in pressure between the bottom and the top of the balloon also decreases, which in turn reduces the buoyant force.

4. At the same time, the balloon is always subject to the force of gravity, which pulls it downwards.

5. At a certain height, the buoyant force equals the force of gravity. When this happens, the balloon stops rising because there is no net force pushing it upwards.

6. Therefore, a hydrogen-filled balloon stops rising after it has attained a certain height in the sky because the buoyant force and the force of gravity balance each other out.