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Can you give me an example where an object (less dense than liquid) is floating in liquid and buoyancy acting on that object is more than the weight of that object ? It is impossible , isn't it? Because for a floating object buoyant force is always equal to the weight of object.

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    $\begingroup$ by definition of buoyant. Archemedes en.wikipedia.org/wiki/Archimedes discovered that the density of an object could be measured by the volume of water its submersion displaced. a first measurement of density without destructing the object. Then logic comes in that if the volume of water displaced has a larger weight than the object , the object floats. $\endgroup$ – anna v Nov 9 '14 at 7:51
  • $\begingroup$ ""if the volume of water displaced has a larger weight than the object, the object floats"" you mean that when you submerge an object totally in water and then if displaced water by that fully submerged object has a larger weight than the object , the object floats . Right? $\endgroup$ – tahsin Nov 9 '14 at 11:45
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    $\begingroup$ yes, it is a method of measuring the density, grms per centimeter cube. $\endgroup$ – anna v Nov 9 '14 at 11:58
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It depends on what you mean by "floating" and "in liquid".

If it is under the liquid and floating upward against friction and other resistance, then yes, the buoyancy is greater than the weight. If it is under the liquid and tethered by a line, then again, buoyancy is greater.

If it is floating on the surface and has been dropped and is bobbing back to equilibrium then buoyancy is greater.

If there are waves, then the buoyancy oscillates between greater than and less than the weight.

For buoyancy to be equal to weight you have to specify an equilibrium condition, like the usual classical mechanics derivation that starts with "assume a homogeneous and isotropic medium" (made out of the same stuff everywhere and having the same properties in any direction).

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  • $\begingroup$ i mean only in water and no other force except buoyancy and weight of the object is acting on the system, then? $\endgroup$ – tahsin Nov 9 '14 at 8:45
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    $\begingroup$ If the air pressure is changing, so will the buoyancy. But under steady state conditions you are correct $\endgroup$ – user56903 Nov 9 '14 at 11:07
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    $\begingroup$ @talsin if the only forces are weight and buoyancy and buoyancy is greater than weight then the object will be accelerated upwards. Similarly if buoyancy is less than weight then it will be accelerated downwards. $\endgroup$ – Peter Green Nov 28 '15 at 15:26

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