When an empty bottle is placed in the water, it will never be able to float upright. Why is this so?

Center of gravity lies above the water surface when the bottle is up-right. Try filling it 1/5 with water. It will prolly float upright now, or sink, in which case you need to pour some water out again.

If the glass were thick enough it would.

Unless you add ballast, which could be water or a weight, the shape is not stable.

 

The righting arm from the changing center of buoyancy does not change fast enough, even at small angles of "heel", to counteract the "tipping moment" that is created.

 

http://en.wikipedia.org/wiki/Metacentric_height

Center of gravity lies above the water surface when the bottle is up-right. Try filling it 1/5 with water. It will prolly float upright now, or sink, in which case you need to pour some water out again.

yes it seems to be logical answer, and explains it.

Edited August 23, 200916 yr by hemantc007

yes it seems to be logical answer, and explains it.

 

It is not necessary (or even a guarantee of stability) for the center of buoyancy of an object to be above that of the center of gravity for it to be in a stable floating position.