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Oceanography/Falling in deep water

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Question
Hi,
What would happen to a weight of say 50 lbs. dropped from a boat floating over the Challenger Deep? Would its rate of descent slow as it got further and further down? Does the extreme weight of the deep water have any effect on something falling through it? I've seen video of submersibles or even fish traversing ocean depths and nothing appears to affect their descent. But that is controlled descent.

Thank you for this service and your time.

Steve
San Diego

Answer
The rate of descent of an object depends of course on the effective weight underwater and the drag it feels due to the viscosity of the water. Your question really has to do with the change in density of the object relative to the density of water as the pressure increases with depth. The short answer is; not much.

The density of seawater is affected much more by temperature and salinity. If we ignore those effects for the moment and just focus on the effects of pressure, the increase in density from 0 to 10,000 meters (32,000 feet or 6.2 miles) is only about 4%. As far as determining the terminal velocity of the object (like that of a skydiver in air), this effect would be hardly noticeable.

You should also realize that the compressibility of the object also needs to be taken into account. Water itself is fairly rigid (resists compression) as far as substances go, so the relative compressibility of the object compared to water could be an important factor. A solid metal ball wouldn't compress much compared to water (I'm guessing; need to know the bulk modulus) but other objects may squash down quite a bit. A favorite trick of oceanographers is to attach a styrofoam cup to an instrument package on a deep cast; it comes back the size of a thimble after the air in the foam is squeezed out.

On the other hand, the compression of seawater in the deep ocean accounts for about a 30 meter lowering of the sea surface compared to an incompressible ocean.

Hope this answers your question.

Oceanography

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randy patton

Expertise

Physical oceanography, surface and internal wave characteristics, ocean currents, fluid mechanics, geophysical fluid dynamics, ocean optics, coastal dynamics, modeling and simulation, data analysis, El Nino and related large scale dynamics Not an expert in marine biology (some in bioluminescence) or chemical oceanography

Experience

26 years as professional scientist for research company working mostly on Navy and other government contracts. Projects included modeling, simulations and data analysis related to Non-acoustic Anti-submarine Warfare (NAASW). Other projects included remote sensing of ocean features, statistical analysis of ship tracks, ocean optics instrumentation development, synthetic aperture radar (SAR) and sonar (SAS).

Publications
Journal of Physical Oceanography, 1984, "A Numerical Model for Low-Frequency Equatorial Dynamics" (with M. Cane)

Education/Credentials
MS Physical Oceanography, MIT, 1981 BS Applied Math, UC Berkeley, 1976

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Currently an Expert for All Experts in Advanced Math

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