Buoyancy is the ability of an object to float in a liquid, determined by the weight of the object and the weight of the displaced water. Archimedes’ principle explains that an object in a fluid is lifted by a force equal to the weight of the fluid displaced by the object. Positive buoyancy occurs when an object displaces more water than its weight, negative buoyancy when an object’s weight is greater than the weight of the liquid it displaces, and neutral buoyancy when the weight of the object and the amount of liquid it displaces are roughly equal. The size and shape of an object affect its buoyancy, and stability in a fluid depends on the position of an object’s center of buoyancy relative to its center of gravity. Engineers use these principles to design boats, ships, and other objects for water activities.
Buoyancy is the ability of an object to float in a liquid. The relationship between the object’s weight and the weight of the displaced water is what determines whether the object will float; while the size and shape of the object have an effect, they are not the main reason 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 variety of water activities, such as boating or scuba diving.
Archimedes’ principle
The mathematician Archimedes, who lived in the 3rd century BC, is credited with discovering much of how floating works. According to legend, one day he was taking a bath and noticed that the more he immersed himself in the water, the more his level rose. He realized that his body was displacing the water in the tub. Subsequently, he determined that an object under water weighed less than an object in air. Through these and other realizations, he established what became known as Archimedes’ Principle:
An object in a fluid is lifted by a force equal to the weight of the fluid displaced by the object.
Positive, negative and neutral buoyancy
An object floating in a liquid floats positively. This means that the amount of water displaced by the object weighs more than the object itself. For example, a boat that weighs 50 pounds (23 kg) but displaces 100 pounds (45 kg) of water will float easily. The boat displaces more water than its weight in part due to its size and shape; most of a boat’s interior is air, which is very light. This explains why huge ocean liners float: as long as the displaced water weighs more than the ships themselves, they won’t sink.
Negative buoyancy is what causes objects to sink. Refers to an object whose weight is greater than the weight of the liquid it displaces. For example, a pebble may weigh 25 grams, but if it displaces only 15 grams of water, it cannot float. If the 50 lb (23 kg) boat was loaded with 75 lb (34 kg) of cargo, it would no longer float because its weight (125 lb or 56.69 kg) is heavier than the weight of the water it displaces (100 lb or 45kg).
It is also possible for an object to be neutrally buoyant. This means that the weight of the object and the amount of liquid it displaces are roughly equal. A neutrally buoyant object hovers in liquid, neither sinking nor floating. A submarine can regulate its weight by adding or expelling water in special tanks called ballast tanks. By properly balancing its ballast, the submarine can hover at various levels below the surface of the water without sinking.
Size and shape
How much of an object’s surface touches water has an effect on its buoyancy. A very large ship has a lot of surface area, which means that the weight of the ship is spread across a lot of water, which pushes up on the ship. If the same ship were in the water with its bow down, it would begin to sink because all the weight is concentrated in a 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 his back, his whole body may remain at or near the surface of the water. When he floats in the water with his feet down, he will sink farther; typically, only the upper body will remain atop the water.
Stability
Stability in a fluid depends on the position of an object’s center of buoyancy relative 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 position of the weight of the object. 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, the object will be stable. If, however, the center of gravity is above the center of buoyancy, as in a ship loaded with cargo high above the waterline, then the object becomes unstable. If the load shifts to one side for any reason, the CG and buoyancy center will no longer align. The ship capsizes as the center of buoyancy tries to rise above the center of gravity.
In the human body, the center of gravity is usually in the navel area. The center of buoyancy is slightly higher, which is why a body tends to float upright with the shoulders and torso over the legs. 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.
Buoyancy in practice
By applying the principles of buoyancy, engineers can design boats, ships and seaplanes that remain afloat and stable in the water. This is true for many other items, such as lifebuoys and pontoons. Almost everything designed for water is based 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 their breath. Also, trying to dive to the bottom of a pool takes effort because the body is naturally buoyant. Divers in particular need to know how to float, hover and sink and often wear extra weights and other gear to help them manage these maneuvers.
Protect your devices with Threat Protection by NordVPN
