A submarine rests tightly against a flat sea bed so that practically no water can enter between its lower surface and the sea bed. Why can the submarine be much more difficult to lift by buoyancy than when it is freely submerged?
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Question

A submarine rests tightly against a flat sea bed so that practically no water can enter between its lower surface and the sea bed. Why can the submarine be much more difficult to lift by buoyancy than when it is freely submerged?

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G1 Physics · Accepted Answer

For a freely submerged body, buoyancy is the resultant of pressure forces over its entire wetted surface. The pressure on the lower surface normally contributes a large upward force. If a submarine is pressed tightly against a flat sea bed and water cannot reach the lower surface, that upward pressure contribution is absent or greatly reduced. The pressure forces can therefore produce much less upward resultant than the ordinary Archimedean buoyant force expected for a fully surrounded submerge…