by Sandra Douglas

Created on: July 16, 2008

Have you ever wondered why a large steel ocean liner floats, but a small steel nail sinks? The answer is surprisingly simple: The steel hull of the ship is formed in a shape that displaces a volume of water that weighs at least the same weight as the ship. If the metal used to make the ship was reshaped into a large block of steel and placed into the same water, it would sink straight to the bottom.

This is known as the principle of buoyancy. The Greek mathematician, Archimedes, was the first person to explain the phenomenon about 2000 years ago as: "Any object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object."

Fresh water weighs 62.4 pounds per cubic foot. If an object weighs more than the weight of the volume of water it displaces, it will sink and is said to be negatively buoyant. If an object weighs less than the weight of the volume of water it displaces, it will float and is said to be positively buoyant. If an object displaces an amount of water equal to its own weight, it will neither float nor sink, but remain suspended and is neutrally buoyant.

So you see, objects don't float on top of the water. They are held up by the force of the water beneath. Archimedes principle holds true for all objects in all fluid. However, an object with a given volume and weight will vary in buoyancy depending on the density of the material in which the object is surrounded. The denser the surrounding liquid, the more buoyant a force will be applied.

Salt water has a higher density than fresh water due to the dissolved salts, so you'll be more buoyant in salt water than in fresh water. That's why it's easier to float in salt water. Most people are positively buoyant in either fresh or salt water. In fact, when floating motionless on top of the water, it is necessary to exhale air from the lungs in order to sink. By exhaling, the volume of the lungs is decreased and less water is displaced resulting in less buoyancy. We can see that changing the volume of an object changes its buoyancy by changing the amount of water that it displaces.

Let's go back to our example of the ship which was reshaped into a block of steel. It is not the weight of an object that determines whether it will sink or float. It is the amount of water that it displaces as determined by the volume of the object. When our block of metal is reformed once again into the large hull of a ship, it now displaces a much larger volume of water. If the weight of the water it displaces is greater than the weight of the ship, it will float.

Buoyancy principles are important in the design of objects that need to float, boating, scuba-diving. Scuba-divers use weight belts to offset the added buoyancy of their equipment and allows them to sink. They also use a BCD or Buoyancy Control Device to manually increase and decrease the volume of air in the device as needed to control their buoyancy. A Scuba diver's goal is to be neutrally buoyant; he wants neither to continually float to the surface nor to sink to the bottom of the ocean floor.

Learn more about this author, Sandra Douglas.
Click here to send this author comments or questions.