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When an object displaces a fluid a force acts on the object which is equal to the mass of the fluid displaced by it (for the logic see here ). But the fact that a net (buoyant) force acts on it is due to the fact that there exists a pressure gradient (i.e., there is more pressure at the bottom than at the top).

Now consider the following:

Suppose that there is an object at the bottom of fluid and the object is placed such that there is no fluid beneath it (rather imagine an outward cylindrical projection from the bottom surface). Since the object has a volume therefore it displaces the fluid and a buoyant force must act on it in upward direction. But also consider the fact that there is no fluid beneath it to push upward (i.e., there is a net downward force).

So

  • Would a buoyant force still act if the object is placed at the bottom the fluid(without any gap for the fluid to enter)?
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  • $\begingroup$ Is the base of your object & the surface it's sitting on perfectly flat and smooth? If so, it'll stick to the bottom. See en.wikipedia.org/wiki/Gauge_block for a related effect. $\endgroup$
    – PM 2Ring
    Commented Nov 26, 2019 at 4:42
  • $\begingroup$ Haha... you are my kind of guy 😄. Had the same thought today. $\endgroup$
    – Lukáš Řádek
    Commented Jul 18 at 18:25

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No, it would not, if there is no gap. On the contrary, the pressure of the fluid would keep it firmly stuck to the surface.

It is the principle behind the whole notion of "suction cup" ! The atmospheric pressure is exactly what makes a suction cup sticking to the surface it is sticking to.

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Buoyancy is lost in the case you describe. It often happens with drill pipe in oil and gas wells with the drill string getting harder to pull out.

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