Neutral buoyancy occurs when an object's average

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

is equal to the density of the fluid in which it is immersed, resulting in the

buoyant force Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

balancing the force of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

that would otherwise cause the object to sink (if the body's density is greater than the density of the fluid in which it is immersed) or rise (if it is less). An object that has neutral buoyancy will neither sink nor rise. Principio di Archimede spinta e peso

scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chr ...

, the ability to maintain neutral buoyancy through controlled breathing, accurate weighting, and management of the buoyancy compensator is an important skill. A scuba diver maintains neutral buoyancy by continuous correction, usually by controlled breathing, as neutral buoyancy is an unstable condition for a compressible object in a liquid.

History

The mathematician

Archimedes Archimedes of Syracuse (;; ) was a Greek mathematician, physicist, engineer, astronomer, and inventor from the ancient city of Syracuse in Sicily. Although few details of his life are known, he is regarded as one of the leading scientis ...

discovered much of how buoyancy works more than 2000 years ago. In his research, Archimedes discovered that an object is buoyed up by a force equal to the weight of the water displaced by the object. In other words, an inflatable boat that displaces 100 pounds (45 kilograms) of water is supported by the same amount of force. An object that floats in a fluid is known as being ''positively'' buoyant. An object that sinks to the bottom is ''negatively'' buoyant, while an object that remains in balance at the same level in the fluid is ''neutrally'' buoyant. Ways to adjust buoyancy were developed to produce equipment such as the inflatable

life jacket A personal flotation device (PFD; also referred to as a life jacket, life preserver, life belt, Mae West, life vest, life saver, cork jacket, buoyancy aid or flotation suit) is a flotation device in the form of a vest or suite that is worn by a ...

, which is filled with gas and helps to reduce a person's average density, assisting in floating and swimming, as well as certain diving equipment (including

submarines A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

and

submersibles A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of ind ...

) which have adjustable volume air chambers to regulate buoyancy.

Uses

Buoyancy is important in many fields. Boats,

ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...

s and

seaplane A seaplane is a powered fixed-wing aircraft capable of taking off and landing (alighting) on water.Gunston, "The Cambridge Aerospace Dictionary", 2009. Seaplanes are usually divided into two categories based on their technological characteri ...

s are

engineered Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...

in a way that ensures that they remain afloat.

Submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s have controllable buoyancy to make them submerge and rise on demand. Many objects were developed with buoyancy in mind, such as life preservers and pontoons. Buoyancy is essential to most water sports. Many swimmers know that there are easy ways to float at the surface, such as lying on one's back or holding a full breath. Buoyancy becomes noticeable when a swimmer tries to dive to the bottom of the pool, which can take effort. Scuba divers work with many buoyancy issues, as divers must know how to float, hover and sink in the water. Scuba divers often wear lead weights to counteract the positive buoyancy of their bodies and gear. Gases are also fluids, and so objects floating in the air may also be trimmed to be neutrally buoyant. A hot-air balloon that is neither sinking nor rising uses the lower density of hot air compared with the density of the surrounding air to produce sufficient upthrust to balance the weight of the basket and its contents.

Microgravity simulation

Neutral buoyancy is used extensively in training

astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...

s in preparation for working in the

microgravity The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms '' weightlessness'' and ''zero-g'', but emphasising that g-forces are never exactly zero—just very small (on the ...

environment of space.

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

and the Russian space program maintain facilities in which suited astronaut trainees interact with mock-up space hardware, with the assistance of scuba divers. At the University of Maryland's

Space Systems Laboratory Space Systems Laboratory is a name commonly used by university laboratories engaged in the research of technologies used for human activities in space. Examples include: * Drexel Space Systems Laboratory at the Drexel University in Philadelphia, ...

, a neutral buoyancy tank is similarly used to evaluate the performance of prototype space robots.

Characteristics

In a still fluid, when an object has neutral buoyancy, there is no net force causing it to float or sink. Any applied force will cause it to move in the direction of the force. If it has no momentum it will remain motionless.

Appearance in nature

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...

controls buoyancy by adjusting the amount of gas in the swim bladder, allowing it to achieve neutral buoyancy at different depths. When a fish's overall density becomes higher or lower than the surrounding water due to volume change of the swim bladder following ascent or descent, it can correct this difference over time by a physiological process involving controlled absorption and elimination of gases via the blood circulation, the gills, and a gland adjacent to the swim bladder. The human brain exhibits approximately neutral buoyancy as a result of its suspension in

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates. CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...

. The actual mass of the human brain is about 1400 grams; however, the net weight of the brain suspended in the CSF is equivalent to a mass of 25 grams. The brain, therefore, exists in nearly neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which would cut off blood supply and kill neurons in the lower sections.

References