Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Neutral bouyancy

Learn more physics!

Most recent answer: 10/22/2007
We have liferafts that are supposed to be neutrally buoyant when in their wrapper at work and got into a discussion about launching a liferaft should the need arise. When we discussed the neutral buoyancy of the liferaft several different definitions came up and we are wondering which is true: The liferaft will not float or sink once in the water but will sit just below the surface of the water (water conditions at optimum - or as tested by the liferaft company) -or- no matter where in the water column the liferaft is, it will stay there. (ie: if it was dropped 10 feet down in the water, it would stay there. Can you help clear up our definition so we can train correctly to the truth?
- Corey (age 35)
Brunswick, Maine USA
"Neutral Buoyancy" means that the density of the object suspended in the water, or what ever liquid, has the exact same density as that of the liquid itself.   So that means where ever you initially place the object, either at the surface or 10 feet below, it will stay there.

Well, as usual, there are a few subtleties involved.   The density of the liquid usually changes as a function of temperature, salinity, etc.   For example it's easier for one to stay afloat in the ocean than in a fresh water lake because salt water is more dense.

As a practical matter, I would suggest that the wrapped life raft package have a slightly higher buoyancy (smaller density) that the water.  In that case the thing will bob to the surface instead of mucking about 10 feet below or even worse, sinking.


(published on 10/22/2007)

Follow-up on this answer.