Ask A Scientist Physics Archive

name         A. E. M.
status       student
age          11

Question -   What is neutral buoyancy?
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A E. -

Buoyancy is the upward force on an object suspended in a fluid - a liquid or a gas.  This 
force is equal to the force of gravity (i.e. weight) on the fluid which is displaced by the 
object.  The more dense the fluid, the greater the force of gravity on the fluid displaced, 
and therefore, the greater the buoyant force on the object.

Your body experiences a buoyant force because you are in the air... a fluid.  In water the 
buoyant force is greater and you appear to weigh less... but likely still have some downward 
force and you will sink unless you start swimming.  In salt water which is more dense it is 
easier to stay on top of the water.  If you were to jump into a vat of the denser mercury 
(please do not, it is poisonous!) you would experience so much buoyant force you would easily 
float.

To your question... neutral buoyancy is when the buoyant force is equal to the force of 
gravity.  The net force will not move the object up or down. Submarines try for neutral 
buoyancy to make traveling up and down in water as effortless as possible.

Larry Krengel
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Neutral buoyancy means that the weight of an object under water is equal to the upward buoyant 
force of the water on the object.  Since the magnitude of the buoyant force is equal to the 
weight of the water displaced by the object, any part of the water itself is in neutral 
buoyancy.  A submarine under water is close to neutral buoyancy.  By admitting water to their 
buoyancy tanks or blowing it out, a submarine can adjust the buoyant force until it just 
balances the total weight of the submarine.  Fish also are
designed to be in close to neutral buoyancy.  They can, in fact, sleep without rising or 
sinking appreciably.

Humans are also close to neutral buoyancy.  When under water, I can adjust my buoyancy by 
expelling air from my lungs until I just start to sink.  If I expel air until I stay at the 
same depth, I am in neutral buoyancy.

Best, Dick Plano...
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