What is Buoyancy?

Buoyancy is the ability of an object to float in a liquid. The relation of the object's weight to the weight of the water displaced is what determines if the object will float; although the size and shape of the object do have an effect, they are not the primary reason why an object floats or sinks. If an object displaces more water than its weight, it will float. Buoyancy is an important factor in the design of many objects and in a number of water-based activities, such as boating or scuba diving.

The mathematician Archimedes, who lived in the third century B.C., is credited with discovering much of how buoyancy works. According to legend, he was getting into a bath one day and noticed that the more he immersed himself in the water, the more its level rose. He realized that his body was displacing the water in the tub. Later, he determined that an object under water weighed less than an object in air. Through these and other realizations, he established what came to be known as the Archimedes Principle:

An object that floats in a liquid is positively buoyant. This means that the amount of water displaced by the object weighs more than the object itself. For example, a boat that weighs 50 lbs (23 kg) but displaces 100 lbs (45 kg) of water will easily float. The boat displaces more water than its weight in part because of its size and shape; most of the interior of a boat is air, which is very light. This explains why massive ocean liners float: as long as the water displaced weighs more than the ships themselves, they will not sink.

Negative buoyancy is what causes objects to sink. It refers to an object whose weight is more than the weight of the liquid it displaces. For example, a pebble may weigh 25 grams, but if it only displaces 15 grams of water, it cannot float. If the 50 lbs (23 kg) boat was loaded down with 75 lbs (34 kg) of freight, it would no longer float because its weight (125 lbs or 56.69 kg) is heavier than the weight of the water it displaces (100 lbs or 45 kg).

It is also possible for an object to be neutrally buoyant. This means that the object's weight and the amount of liquid it displaces are about the same. A neutrally buoyant object will hover in the liquid, neither sinking nor floating. A submarine can adjust it weight by adding or expelling water in special tanks called ballast tanks. By properly balancing its ballast, the sub can hover at various levels under the surface of the water without sinking.

How much of an object's surface touches the water has an effect on its buoyancy. A very large ship has a lot of surface area, which means that the ship's weight is spread out over a lot of water, all of which is pushing up on the ship. If the same ship was in the water with the bow pointing down, it would start to sink because all of the weight is concentrated in one small area, and the water it is displacing weighs less than the weight of the ship.

A common example used to demonstrate this is a person floating in water. If the person floats on her back, her entire body can stay at or near the water's surface. When she floats in the water with her feet down, she'll sink farther; typically, only her upper body will stay at the top of the water.

Stability in a fluid depends on the location of an object's center of buoyancy in relation to its center of gravity. An object's center of gravity is the point in the object where all of the object's weight appears to be concentrated; it can also be thought of as the average location of the object's weight. The center of buoyancy is the center of gravity of the water that the object has displaced. This is not in the water, but in the object floating on it.

When the center of buoyancy is directly above the center of gravity, then the object will be stable. If, however, the center of gravity is above the center of buoyancy — as in a ship that is loaded with freight high above the water line — then the object becomes unstable. If the freight shifts to one side for any reason, the center of gravity and the center of buoyancy will no longer line up. The ship will tip over as the center of buoyancy tries to rise above the center of gravity again.

In the human body, the center of gravity is usually in the area of the navel. The center of buoyancy is slightly higher, which is why a body tends to float upright with the shoulders and torso above the legs. Turned upside down, where the legs are above the torso, the body's center of gravity is above the center of buoyancy. This makes the body unstable, and the position can only be maintained through effort.

By applying the principles of buoyancy, engineers can design boats, ships, and seaplanes that remain afloat and stable in water. This is true of many other objects, such as life preservers and pontoons. Just about anything designed for water relies on an understanding of these principles.

Many swimmers know that there are ways to make their bodies more buoyant, such as lying on their backs or holding a full breath. In addition, trying to dive to the bottom of a pool takes effort because the body naturally floats. Scuba divers in particular need to know how to float, hover, and sink, and they often wear extra weights and other gear to help them manage these maneuvers.