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Blue Sky Science: Why do some logs float and some sink?

Holden Taggart

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Why do some logs float and some sink?

Whether an object floats or sinks in water is determined by the ratio of its weight compared to its volume. If an object of a certain volume weighs more than an equal volume of water, it sinks because the water can’t hold it up. If an object weighs less than an equal volume of water, it floats because the water can support its weight.

So does one cubic inch of wood weigh more or less than one cubic inch of water? The answer depends on the type of wood and determines whether that wood would float or sink.

This ratio between weight and volume is called density. An object that is less dense than water can be held up by water, and so it floats. An object that is more dense than water will sink.

The logs that float are less dense than the logs that sink.

Testing out a few samples of wood illustrates this. A piece of cedar, which is rather light, will easily float on top of water. Oak, on the other hand, is a much heavier piece that’s often called a hardwood. It still will float, but some of the wood is submerged into the water.

A third type of wood, called ipe, is from a tree that grows in Central and South America. Unlike the other two examples, this dense wood sinks all the way to the bottom when placed in water.

But why is some wood less dense than others?

The density is determined by the size and number of openings in the wood. These openings allow tree sap to run through the tree. Thick sap requires big openings to flow through, while thin, watery sap needs only small openings.

Wood that is less dense and floats has big openings. Wood that sinks has very tiny openings.

About Blue Sky Science

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research. The questions are primarily posed by visitors attending Discovery Building events.

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