Density of wood and Iron

[holyduckie]

A block of wood with a piece of iron on top is floating on water. A mark is made on the wooden block to indicate the water level. If the iron were suspended beneath the wood and the whole system is still afloat, where will the new water level be and Why? ( with respect to original mark)

[gen2]

The new water level would be lower down the block of wood, ie less of the wood would be submerged.

In both cases the combination of wood and metal is floating so both are displacing the same volume of water. This displaced water weighs the same as the combined weight of wood and iron. In the first example with the iron on top, only the wood is displacing this water. In the second case, the iron is below the water and therefore displaces its own volume of water so the wood has to displace less to come to the same total.

[Mudman]

Does this mean, that these 'jack up' Oilrigs... (http://uk.search.yahoo.com/search/images/view?back=http%3A%2F%2Fuk.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Djack%2Bup%2Brig%26ei%3DUTF-8%26fr%3Dslv1-%26b%3D121&w=443&h=383&imgurl=www.fels.bg%2Foffshore_proj%2Fvicksburg.gif&rurl=http%3A%2F%2Fwww.fels.bg%2Foffshoreprojects6.html&size=23.2kB&name=vicksburg.gif&p=jack+up+rig&type=gif&no=135&tt=224&ei=UTF-8) Actually 'rise' from the water as the legs are being lowered to the seabed?.. (just curious)

[stanleyman]

The level would be the same, only the draft and centre of gravity is altered because the total displacement is the same. The bouyant wood displaces the same if it is pushed or pulled down by the iron. The iron has no bouyancy itself therefore its displacement is added to the wood in any position and cannot alter the water level unless the iron has some inbuilt bouyancy. A boat for example has the same water level regardless of the same internal or external ballast.

[stanleyman]

Yes mudman, the floating oilrig is jacked up from the water surface by the extending legs that are placed on the seabed to give a rigid platform away from stormy waters. They can be moved to different sites.