Re: How does wood float in water & why does some wood float higher than other?

Wood floats in water because it is less dense (lighter) than water. That is, a 
unit volume of wood has less mass (and weight) than the same unit volume of 
water. Basically, objects sink into the water until they displace (take the 
place of) a volume of water equal to their own weight. If the entire object 
displaces a volume of water which weighs less than the object (a stone or a lead 
weight, for example) the object will sink.

Wood is made of dead plant cells and contains many open spaces. This is why it 
is so relatively light when it is dry. Fresh (wet) wood will float but not as 
high as the same wood would if dry. This is because the spaces in the wood are 
filled with water and the wood is heavier (more dense).

Even so, the material of which the cells are made is still lighter than water 
and so wood floats even when wet. Sometimes wood becomes so saturated with water 
(even the exceedingly small spaces in the cell walls themselves are filled with 
water) that it sinks.

Different types of wood will have different proportions of the various cell 
types which make up the wood and different proportions of the various substances 
which make up the cell walls of the wood cells (cellulose and lignin). This 
probably accounts for the different floating properties of various kinds of 
wood.

To learn more about the properties of different kinds of wood you should try a 
good library (especially one at a college or university) or do a web search. I 
did a search on "wood and density" and got lots of hits. One of the more 
interesting ones is listed below. Good luck with your study of wood and its 
properties.

http://www.granadatv.com/madscience/fred.html