Why do things Float?
Buoyancy is partially responsible for the ancient art of naval warfare, the floating of the simplest wooden raft in back yard ponds, and the sailing of the largest ocean liners in the world. Lack of buoyancy caused the greatest oceanic disaster in history, the sinking of the Titanic. Buoyancy also saved individual lives in the same tragic incident, as a few lucky survivors not only observed, but participated in. What is buoyancy?.
Buoyancy is the tendency or ability of any object to float in a liquid, or rise in a gas. It is a curiosity that the materials involved do not themselves create buoyancy. Steel sinks in water, yet a steel-hulled ocean liner floats easily. A concrete block sinks, but cement-hulled yachts float elegantly. Some individual people can float, while others sink. Why? Let us examine the physics behind the phenomenon known as buoyancy.
Each individual material in the universe has mass and volume. When mass and volume are expressed in a ratio, the concept of specific gravity is created. What is specific gravity? Specific gravity is an international standardized measurement of weight (or mass) per unit of volume, usually expressed as compared to that of water . Each cubic centimeter (c.c.) of water weighs 1.0 gram. A metal that weighs 5 grams per cubic centimeter and is 5 times heavier than water would therefore have a specific gravity of 5.0 and sink quickly in water-a lack of buoyancy.
It is the physical characteristics of mass and specific gravity and the resultant differential between the specific gravity of different materials that really causes the phenomenon known as buoyancy, or floating. As Archimedes principle may be paraphrased, when an object is placed into water, the buoyancy provided by the water is precisely the weight of the volume of water displaced, regardless of the shape of the object. Archimedes was reported to have said " Eureka!" when he made that discovery, and no wonder, for it is an amazing concept.
In simple, practical application, if the weight of an object exceeds the weight of the water it displaces, the object will sink. If lighter, it will float.
Let us examine what actually causes buoyancy. When an object of any shape and specific volume is placed in water, it attempts to displace exactly the same volume of water. Coincidentally, the weight of the water displaced equals the weight of the object. The weight of the water displaced provides buoyancy, or flotation for the object, but only to the limit of the weight of the water displaced.
In the simplest example, if any material has a lower specific gravity than 1.0, it will float in water. The specific gravity of water equals 1.0 and if the object also displays a specific gravity of 1.0, the object will neither float, or sink, but display neutral buoyancy.
Any combination of physical materials that in total maintains an average specific gravity of 1.0 will display neutral buoyancy. If the combination of materials in the object has an average specific gravity lower than 1.0, it will float, therefore a steel-hulled or cement vessel will float because the volume of the vessel includes a very high percentage of air, which is far lighter than water.
As an example, consider that typical steel is 7.8 times as heavy as water. With a S.G. of 7.8, a hollow steel cube made of 1 cm thick steel sheeting on all six sides will weigh approximately 46.8 kilograms. When thrown in the water, the box tries to displace it's volume in water, but cannot, because it can only displace it's own weight of 46.8 kg in water. The volume of the closed cube is 1,000 liters.
To sink, the cube would have to displace 1,000 liters, or 1,000 kg. of water, therefore the cube will float quite high in the water because of the natural buoyancy. In the same example, the natural buoyancy causing the cube to float can be calculated as: 1,000- 46.8 = 953.20 kg of uplifting force, far exceeding the weight of the cube.
Most people think of buoyancy only as involving water, but other applications of buoyancy are also common. Majestic, coloured hot air balloons rise magically into the air because of buoyancy . The heated air in the balloon is less dense, or has a lower specific gravity, than the colder air surrounding the balloon, so it rises by virtue of buoyancy. Dirigibles and weather balloons rise into the air because they are filled with helium, a gas which is lighter than air, causing buoyancy. In nature itself, columns of buoyant, heated air naturally rise, and can cause hurricanes and tornadoes. Everywhere in nature, buoyancy is in action.
Now you know what causes buoyancy. If you are a swimmer and float easily as some people do, you are not only observing buoyancy at work, but actively demonstrating one of nature's finest capabilities.