# Law of floatation

1. Aug 15, 2010

### starwars32

Hi.. I understand that if the ice is floating in water, if it melts, the volume of the water will not change as we can apply the law of floatation. But if there is a stone now on top of the ice.. why is it that the water level will decreases when the ice melts. can we like visualise in this way: when the ice melts the stone falls into the water? then the water level increases or decreases? Can we also explain that since the ice is floating with the stone on top.. the overall weight of the stone plus the ice = weight of the water displaced by law of floatation? Can anyone explain? Thanks

2. Aug 15, 2010

### Jobrag

Because a stone sitting on top of a block of ice is displacing it's own mass of water, however a stone sitting on the bottom of the sea is only displacing it's own volume of water.

3. Aug 15, 2010

### Bob S

There is another equally important law of flotation, namely the rotational stability of the floating object. If a stone is frozen into a block of ice above the waterline such that the center of buoyancy is below the center of gravity, the block is often rotationally unstable.

Case in point. The 3-mast battleship Vasa, ostensibly the largest and most beautiful of all warships ever built, was launched at Stockholm, Sweden in 1628. Less than 2 km from port on its maiden voyage, it capsized and sank. The ship's high center of gravity, above the center of buoyancy, was due in part to the two gun decks overloaded with ~ 64 bronze cannons. The inquest into the sinking attributed the capsizing to "an act of God". See

http://en.wikipedia.org/wiki/Vasa_(ship [Broken])

Bob S

Last edited by a moderator: May 4, 2017