Why doesn’t the incredibly high water pressure slow them down at all?
I don’t understand what pressure has to do with it. Pressure is the same in every direction. For every molecule pushing you back there is another one pushing you forward. Viscosity is what would slow you down.
Sailor is correct. It is the density/viscosity of a liquid which will determine the ease at which objects pass through it. Liquids do not compress well even under extreme pressure so even at extreme depths its density (and viscosity) is probably the same as at the surface.
And, according to what I’ve gathered, water viscosity increases with pressure - this being one of the “anomalies” of water. Note that this is true at temperatures below 33 degrees C, and pressures of less than 150 MPa. (The water pressure in the Mariana Trench is about 110 MPa.)
Oh no, not again…
Aaaaargh!
“water viscosity decreases with pressure”
My apologies.
The density is actually slightly higher at great depths, as this page shows, but the differences are small. Incidentally, I believe the fact that water in the depths is colder than that at the surface is the predominant factor.
So the issue if being “under pressure” has nothing to do with impeding movement because pressure is equal on all sides. I suppose I was thinking of being under pressure as a sort of “squeezing” that would affect potentially movement somehow, but if you are saturated with water, pressure is equal on all surfaces and the water is not very compressible, there will be no interference with movement.
Thanks for the clarity!
I believe the term you are looking for is ambient pressure. At depth this would be absolute pressure which is the sum of atmospheric and hydrostatic pressure.
The quantity that actually matters is the Reynolds number. This is a unitless quantity that indicates the “quality” of fluid flow. It is defined as R = d v L / u, d = density, v = velocity, L = length, u = viscosity.
A high Reynolds number indicates a very “thin” flow. A low number indicates a very “thick” flow.
As an example, when engineers put a model airplane in a wind tunnel, they carefully match the wind speed so that the Reynolds number of the model is the same as the Reynolds number of the real jetliner at its cruising speed. When the Reynolds numbers match, the fluid flow around the model and the jet will be the same, even if the density and airspeed are different.