Neutral buoyancy

Neutral buoyancy

Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink. An object that has neutral buoyancy will neither sink nor rise.

In scuba diving, the ability to maintain neutral buoyancy through controlled breathing is an important skill.



The mathematician Archimedes discovered much of how buoyancy works almost 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 buoyed up by that same weight of support. An object that floats in the water is known as being positively buoyant. An object that sinks to the bottom is negatively buoyant, while an object that hovers at the same level in the water is neutrally buoyant. Scientists later discovered ways to manipulate buoyancy and developed equipment such as the life jacket, which is filled with compressed air and helps to lower a person's average density, assisting in floating and swimming, as well as certain diving equipment (including submarines and submersibles) which have air chamber similar to swim bladders in order to regulate depth.


Buoyancy is important in a surprising number of fields. Designers and engineers must design boats, ships and seaplanes in a way that ensures that they remain afloat. In the case of submarines, experts developed ways to make them sink and bring them back to the surface. Many objects were developed with buoyancy in mind, such as life preservers and pontoons.[1]

Additionally, buoyancy is very important in a number of water related sports. Many swimmers know that there are easy ways to float at the surface, such as laying 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. In fact, scuba divers often wear lead weights to counteract the positive buoyancy of their bodies and gear.

Microgravity simulation

Neutral buoyancy is used extensively in training astronauts in preparation for working in the microgravity environment of space. NASA 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, a neutral buoyancy tank is similarly used to evaluate the performance of prototype space robots.


When neutral buoyancy is taking place, it appears as though the object/substance is floating in the middle of the fluid, or somewhere in between the bottom and the surface.

Appearance in nature

A fish's swim bladder manipulates neutral buoyancy by controlling the amount of water and air in the swim bladder, allowing it to swim at different depths. This is achieved by having an average density that is lower than the surrounding water, with the density of the fish being counter-acted by the density of the air in the bladder.

On an interesting note, we all deal intimately with the phenomenon of neutral buoyancy every day--with our brains, which exhibit neutral buoyancy as a result of their suspension in cerebrospinal fluid. 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 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.

Creating neutral buoyancy

Creating neutral buoyancy is simple. Find an object, and two fluids, one that is more dense and one that is less dense. In the first fluid, the object will float. In the second, it will sink. However, when you mix together the two liquids, it will be somewhere in between, because the force of gravity pushing down on the object equals the force of buoyancy on the relative density of the object, causing it to land in the middle of the fluid. There are exceptions to this rule however, as is the case with insoluble liquids. Because they do not mix well, one stays suspended on top of the other, and any object dropped onto the liquid will land according to its density.

Density of everyday substances (in g/cm3)

Water Vinegar Ice Salt Water Milk(Skim)
1.00 0.78 0.92 1.03 1.033


See also

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