Question

Assertion: A balloon with hydrogen stops rising after it has attained a certain height in the sky.
Reason: As height increases, density of air decreases resulting in increase of buoyant force.
A. Both Assertion and Reason are correct and reason is the correct explanation of assertion.
B. Both Assertion and Reason are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect
D. Both Assertion and reason are incorrect

Answer 1

Buoyancy is defined as the force that causes the objects to float. It is a force which is exerted on an object that is partly or wholly immersed in a fluid. An object when immersed in a liquid experiences an upward force which is known as buoyant force. This force is also known as the upthrust.

Complete answer:
When the air displaced by the balloon is greater than its weight, then an upthrust or buoyant force is applied on it in an upward direction and it rises. As the balloon rises, the density of the air decreases, and the buoyant force or the upthrust by the surrounding air decreases because of the decrease in the density of the surrounding air. At this same point, the weight of the balloon becomes equal to upthrust and it stops moving up. Thus, as the height increases, density decreases. So, buoyant forces decrease.
Therefore, Assertion is correct but reason is incorrect.

Hence, option (C) is the correct answer.

Note:
When an object is submerged in a fluid, it experiences an upward force. This force on the object is applied by the fluid and it makes it rise up this is known as the force of buoyancy. The scale of this force is expressed in the amount or weight of the liquid displaced. The centre of buoyancy is defined as the point where the force of buoyancy is applied or the point on the object where the force acts. The SI unit of buoyant force is Newton.