When a body is placed in a liquid and displaces its own weight of the liquid, the up thrust and the weight of the object are now in balance. Why doesnt the body sink or rather continue its motion( Downwards or sinking) in a straight line?
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1$\begingroup$ Welcome on Physics SE :) Does this en.wikipedia.org/wiki/Buoyancy help you? Also, it is always kind of hard to answer "why does XY not happen" questions if the "experimental" setup of the thought experiment are not very concisely stated and I am having a bit of trouble with that here - could you elaborate a bit? :) $\endgroup$– SanyaCommented Sep 21, 2016 at 17:18
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1 Answer
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The net force on the floating object is zero. So by Newton's 1st Law the object is either at rest or in a state of uniform motion (constant velocity).
The floating object is in a state of stable equilibrium. If the object moves down a little, the buoyancy force increases whereas its weight remains the same - then there is a net upward force on it. If the object moves up a little, the buoyancy force decreases - now there is a net downward force on it. Whenever the object moves away from its equilibrium position, there is a net force on it towards the equilibrium position. So the object cannot be in uniform motion.
If the object is dropped into the liquid, it will keep moving down when it reaches the equilibrium position (where upthrust = weight) but it will then slow down and move back up. It will oscillate about the equilibrium position until its kinetic energy is transferred to the liquid in the form of waves and circulating currents and molecular motion, ending at rest in the equilibrium position.
answered Sep 21, 2016 at 17:47