Why do things float?

(Lansing State Journal, March 11, 1992)

Question submitted by: Steven Thompson of Williamston Elementary 

All objects are made up of tiny units called atoms, each of which has mass.  This means that as gravity acts on the atoms, they have weight. Various objects or materials contain different numbers of atoms. And therefore their masses filling an amount of space will differ.

We call the amount of mass in a given amount of space the density.  For instance, we could measure the mass of fresh water (like water from a stream) in a container.  We could then determine the density if fresh water.  However, if we filled the same container with salt water, it would have more mass because the ocean water has the mass of pure water plus the mass of the salt.

Since there is more mass in the same volume, ocean water has a greater density then fresh water.

For something to float, it must have a density less than in what it is floating.  This means that steam, which is less dense than air, rises (or floats) in the air.  This is also why clouds float; the water molecules which make up the cloud are very spread out, and therefore the cloud is less dense than the air and it floats.

It may seem odd that an enormous ship can float when steel is more dense than water.  However, if you look at the entire ship you see that there are many places filled with things less dense than water, like air.  This means that the density of the entire ship is less dense than that of water and it floats.

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