The Ship Is Mostly Air

Drop a steel nail into a bucket of water. It goes straight to the bottom, and nobody is surprised, because everybody knows steel sinks.

Now go and look at a cargo ship. Forty thousand tonnes of steel, sitting on the water, not sinking.

Both of those are true. Steel sinks and steel ships float, and there is no trick and no exception hiding anywhere. The way out is to notice that "what is it made of?" is the wrong question.

A ship is not made of steel. A ship is made of steel and air, and the air is most of it.

Imagine the hull as a box. The steel walls are a few centimetres thick; the space they enclose is the height of a building. Weigh the whole thing — steel plus air, everything inside the hull — and divide by the whole volume, and the answer comes out less than water. That is the number that decides. Not what the walls are made of. The average of the whole object, air included.

And here is the part you can check yourself, tonight, with nothing:

Take a lump of clay or dough and drop it in a bowl of water. It sinks. Take the same lump out, press it into a little bowl shape with your thumbs, and float it.

Same clay. Same grams. You changed nothing except how much space it takes up.

That is not a model of how a ship floats. That is how a ship floats.

The quantity that has been doing all this work has a name, and you have met it before without being told what it was: in lesson 23 it decided which liquid sat on top of which, and it is why oil floats on water. It is called density, and this lesson is where it gets pinned down properly.

By the end of this lesson you'll be able to:

• define density as mass ÷ volume, and calculate it
• predict whether something floats or sinks by comparing its density with water's 1 g/cm³
• explain why a steel ship floats when a steel nail does not
• explain what displacement is, and why a loaded ship sits lower
• say why "floats" and "sinks" are always statements about two substances, never one