Question

why is it so that an object will experience buoyant force only when it is fully or partially submerged​

Answer 1

Object will experience buoyant force only when it is fully or partially submerged, it is because buoyant force or upward force is always exerted by the fluids (gases & liquids). So if it is fully or partially submerged in the fluids then obviously, the upward/buoyant force will act.

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Answer 2

Answer:

when the body is submerged it displaces the liquid. liquid does not to vacate it's place. So it applies force.