Asked • 07/31/19

What is the basic reason behind buoyancy?

Yesterday my friends and I were solving this easy-looking question: > A metal cube is placed in an empty vessel. When water is filled in the vessel so that the cube is completely immersed in the water, the force on the bottom of the vessel in contact with the cube: > (A) Will Increase > (B) Will Decrease > (C) Will remain the same > (D) Will become zero At first we thought the answer to be (B) considering the buoyant force acting opposite to the weight of the cube. But then, a question popped up in my mind: "What causes this buoyant force even if no fluid was *actually* displaced?" So I referred to Archimedes' Statement: >Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. — Archimedes of Syracuse But according to the question, **no such fluid was displaced** by the block! So, here are my questions: 1. What is the basic **reason behind buoyancy**? Is it the pressure difference in fluids or the 'liquid-displaced' theory? 2. What must be the answer to the question (That I was solving) ? 3. Considering the case that you may say- "The reason behind buoyancy is the pressure difference in the fluids"; Will there be no buoyant force applied to the iron cube as there is no fluid layer present below the cube i.e. no pressure difference? (Considering the iron cube is VERY dense and thus not allowing any fluid layer to push its way below it) 4. Or you might add another statement to the above question that-"Yes, buoyant force is applied as there is always a fluid layer present below it..." .Please do explain yourself. 5. If your answer is according to the Principle given by Archimedes of Syracuse , will there be no buoyant force on the cube as the cube didn't actually displace any fluid? Just like the case where you may consider it to be a part of the vessel itself.

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