How Ships Float and Submarines Dive Quiz
Why things float
When something is put in water, it pushes some water out of the way. The water pushes back up with a force equal to the weight of the water that was moved. This is Archimedes' principle, named after the ancient Greek thinker said to have worked it out in his bath.
If an object is lighter than the water it pushes aside, the upward push wins and it floats. If it's heavier, it sinks.
That's why it's not just what something is made of that matters, but also its shape. A lump of steel sinks, but steel spread into a hollow hull pushes aside a lot of water while staying light for its size.
How steel ships float
A ship's hull is mostly full of air. Taken all together, steel plus air, the ship is less dense than the water, so it floats even though steel on its own would sink.
The more cargo a ship carries, the deeper it sits, because it must push aside more water to stay up. Salt water is denser than fresh water, so a ship floats a little higher at sea than in a river.
Ships have a load line painted on their sides, often called the Plimsoll line. If the water rises above it, the ship is overloaded and unsafe.
How submarines dive and surface
A submarine can choose whether to float or sink. It has large ballast tanks. To dive, it opens vents and lets seawater flood in, so the submarine becomes heavier than the water it pushes aside and goes down.
To surface, it blows compressed air into the tanks, forcing the water out. Now it's lighter again, and it rises.
Small fins called diving planes help steer it up or down as it moves. Near the surface, the crew can look out through a periscope, a tube with mirrors or prisms that peeks above the waves.