What Makes a Ship Float?
A small steel ball dropped into water sinks immediately. A ship weighs thousands of tons, yet it floats on the ocean’s surface. A ship floats because it is buoyed, or held up, by the water. The ship’s big hollow hull pushes aside a volume of water equal to the ship’s weight.
The buoyancy produced by this displacement of water supports the ship, keeping it afloat. This rule of buoyancy is called “Archimedes’ Principle,” after the Greek mathematician who discovered it. A solid steel ball sinks because it weighs more than the water it pushes aside.
Heavy ships can float, because the water they are floating in pushes upward against them. This pushing force is called up thrust. Up thrust from the water makes a hollow hull float in the same way.
The higher the density of water, the greater the up thrust. This means that ships float slightly higher in salt water, since it is denser than fresh water.
The size of the up thrust depends on how much water the object pushes out of the way. When you put an object in water and let it go, it settles into the water, pushing liquid out of the way.
The further it goes in, the more water it pushes away and the more up thrust acts on it. When the up thrust becomes the same as the object’s weight, the object floats.