When an object is wholly submerged and displaces its own volume of fluid and the weight of the fluid balances with the weight of the object, the object floats. But following Newton's first law, if the resultant force = 0, the object should stay at rest or uniform motion in a straight line. Why doesn't the object sink with a uniform motion in a straight line?
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$\begingroup$ A submarine is carefully designed to weigh just as much as the water it displaces. The net force on it is $0$. It does not sink. A rock sinks because it weighs more than the water it displaces. The net force is downward. $\endgroup$– mmesser314Commented Apr 26, 2017 at 13:07
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1 Answer
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This is because of drag.
Any movement in the fluid will actually generate additional drag force upwards, which would make it unbalanced net force.
It's possible (and fairly common) to see something fall through a liquid at a constant speed when the buoyant force isn't strong enough to overcome the weight of the object. As it begins to accelerate downwards, the drag force increases (drag varies with velocity), at some point, the drag force will be perfectly equal to the gravatational force.
This is called terminal velocity and can be seen fairly easily dropping something into a fishtank for example. It also applies in gasses, for instance freefalling humans in Earths atmosphere have a maximum terminal velocity based on their orientation and the air properties, they do not keep accelerating (and a parachute increases the drag to lower the terminal velocity to something that wont hurt on landing).
