Mark Pearson, age 12, of Window Rock, Arizona, for his question:
Why does wood float?
Behind the scenes there is an invisible force called buoyancy. It causes wood to float on water. It causes a cork to bob high on the waves and explains why a ship sinks lower when loaded with a heavy cargo. Like all other forces of nature, buoyancy is governed by precise rules. These rules determine why a certain substance floats and also how much of it is above the water level.
Some of us like to think that modern folk are a lot smarter than their ancestors. Maybe this is so. But it so happens that today's Andy question was solved some 20 centuries ago by a wise old Greek. His name was Archimedes and what he discovered about floating and non floating objects is called the Principle of Archimedes.
We can state this in a nutshell, as follows. A body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. Naturally a large idea stuffed into a nutshell is quite crowded and perhaps hard to grasp. Please do not feel discouraged. It can be spread out, examined and explained in detail. After all, what was understandable in ancient Greece can be grasped by the children of modern man.
In such matters it helps to use an example and it's only fair to use the one suggested in today's question. The body is a piece of wood and the fluid is water. We all know that an ordinary log floats on a stream. The principle goes on to tell why using four key words to state one equation.
The floating body is buoyed, or held up, by an invisible force. The strength of this buoyancy force is related to weight. Actually it is related to the weight of wood and the weight of water. In this case, the buoyancy equals the weight of water that the floating wood displaces, or pushes out of the way.
Perhaps we can simplify the whole thing and say that wood floats because it is lighter than water. But there is far, far more than this to the principle of buoyancy. We want to know why a cork floats higher in the water. Also why a chunk of steel sinks, yet a great steel ship sails on top of the sea.
These details involve volume and density. Volume is the amount of space occupied, in this case by a wooden log and some displaced water. Density is how much a certain volume of this or that weighs for its size. This depends on atoms and molecules and how they are packaged together in different substances.
Air is less dense than water because its molecules are much farther apart. A dry log is made of boxy cells filled with air, so the over all density of the log is less than water.
An even more porous cork floats with only a quarter of its volume submerged. Water is four times denser than ordinary cork which is called specific gravity and leads to another Andy question. In any case, any object floats because it is buoyed up by a force that equals the weight of the amount of water it displaces.