When a solid body is partially or completely immersed in a fluid, the fluid exerts an upward force on the body. The magnitude of the force is equal to 1. the mass of the body 2. the weight of the displaced fluid by the body Which of the above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Correct answer
Explanation
The correct answer is B, 2 only. By Archimedes' principle, the upward force, called buoyancy or upthrust, equals the weight of the fluid displaced by the body.
Archimedes' principle states that a body wholly or partly immersed in a fluid feels an upward force equal to the weight of the fluid it pushes aside. This is why a body seems to lose weight in water, and the loss of weight equals the buoyant force. If the buoyant force on a fully immersed body is greater than its weight, it rises; if less, it sinks. A floating body displaces fluid equal to its own weight, which is the law of flotation behind ships, hydrometers and submarines.
A is wrong because the buoyant force does not depend on the mass of the body; two bodies of equal volume but different mass feel the same upthrust. Besides, a force cannot equal a mass, which is measured in kilograms. C is wrong because statement 1 is incorrect. D is wrong because statement 2 is exactly Archimedes' principle.
Exam tip: buoyant force = weight of fluid displaced (Archimedes); floating body displaces its own weight of fluid.
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