[Physics] Does buoyant force vanish if there is no liquid below an object

buoyancyfluid-statics

A cube of oak wood with very smooth faces normally
floats in water. Suppose you submerge it completely and press
one face flat against the bottom of a tank so that no water is
under that face. Will the block float to the surface? Is there a
buoyant force on it? Explain.

My thoughts:

  1. Buoyant force is a well known consequence of pressure difference across fluids, and if there's no fluid under the object then there should be no buoyant force?

  2. Practically speaking, the block should rise to the surface again.(I tried it)
    However, according to my understanding of fluid mechanics, as long as there is no net upward force on the block, it can't start moving upwards

Could someone please explain this, and also tell whether buoyant force really does vanish if the block is pushed to the bottom of the vessel?

Thanks!

Best Answer

The question is posed as a thought experiment. So arguing with any complications due to limits of technology doesn't help in answering it.

The answer is yes. When you evacuate the space between the object and the bottom of the water vessel, there is no buoyancy on the object (which goes upward). There is however a net resultant down force on the object due to the water pressure difference between the top and the bottom of the object, pushing it down.

What complicates the matter (adding a little more reality to the thought experiment) is that the water to the left and right of the object exerts a pressure near the bottom which will make water flow underneath it. This then creates the pressure difference that causes the object to move up. But that's not really hydrostatics anymore.

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