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Suppose you drop an object into a body of fluid (on a planet with constant gravity). Suppose the fluid is pure (as in, there are no solid granulates or bubbles present in it), and finally suppose that there are no waves in the fluid, and that its density is constant everywhere.

Is it possible for said object to start sinking, but eventually slow down and completely stop sinking further (as in, it will stop and remain suspended in the liquid without hitting the bottom)?

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up vote 5 down vote accepted

I see this is a follow-up post to Suppose a hollow metal sphere filled with helium is dropped in a body of water

Well, the situation you are describing is possible if the object in question can change its average density while in the water. It will stop sinking when $\rho_\text{average} = \rho_w$.

In fact, there's a vessel that uses this exact principle: the submarine. When it wants to submerge it fills up its ballast tanks with water, becoming more dense in the process. Then it sinks. When it wants to stop (and/or rise) it forces air into the ballast tanks which pushes the water out, so it becomes less dense, and it stops sinking/rises.

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So basically, its not possible unless an external force is applied to the object while it is in the fluid (how else will its density change?). – chubbycantorset Jan 15 '13 at 7:08
Yes, as long as there is a net density difference, there will be a net force due to gravity. – Bernhard Jan 15 '13 at 7:52
Perhaps it can interest you that if you consider friction, even if the object will continue sinking, at a certain moment friction and gravity will be equal and opposite, leading to constant speed (sedimentation speed). – Bzazz Jan 15 '13 at 12:21

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