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Christ · 05-16-2010, 04:58 PM

Just to clarify something -

The shrimpy dude, as well as bacterial flagellum, exist in incompressible fluids. Trust me - that helps.

They have to spend less energy to begin moving because the medium is already compressed. In air (back up here, where we drive) the medium isn't already compressed... this means that ALOT of energy has to be "wasted" to compress the medium so that it's density is correct to propel the vehicle through space. This has to happen at the same rate of speed (slightly faster, actually) that the vehicle wishes to progress.

Get in the water - now wave your arms quickly from front to back, while standing completely immersed. I bet your body moved, did it not? You could feel the drag of the water against your arms, driving you forward. Now do the same thing in open air. The only movement you feel is inertia from your arms' movement. The compressible air around you will flow out of the way before you can compress a column of it tightly enough to provide propulsion.

Hold a fish behind it's gills in water while it attempts to get away... most average sized fish will be able to either pull away from your hand, or force you to move as they do.

Do the same in air... that fish can't compress the air fast enough to do anything more than wiggle in your hand. It can't propel itself at all.

You can't directly compare things that exist in incompressible medium with those that exist in the opposite.

05-16-2010, 04:58 PM	#166 (permalink)
Christ Moderate your Moderation. Join Date: Nov 2008 Location: Troy, Pa. Posts: 8,919 Pasta - '96 Volkswagen Passat TDi 90 day: 45.22 mpg (US) Thanks: 1,369 Thanked 430 Times in 353 Posts	Just to clarify something - The shrimpy dude, as well as bacterial flagellum, exist in incompressible fluids. Trust me - that helps. They have to spend less energy to begin moving because the medium is already compressed. In air (back up here, where we drive) the medium isn't already compressed... this means that ALOT of energy has to be "wasted" to compress the medium so that it's density is correct to propel the vehicle through space. This has to happen at the same rate of speed (slightly faster, actually) that the vehicle wishes to progress. Get in the water - now wave your arms quickly from front to back, while standing completely immersed. I bet your body moved, did it not? You could feel the drag of the water against your arms, driving you forward. Now do the same thing in open air. The only movement you feel is inertia from your arms' movement. The compressible air around you will flow out of the way before you can compress a column of it tightly enough to provide propulsion. Hold a fish behind it's gills in water while it attempts to get away... most average sized fish will be able to either pull away from your hand, or force you to move as they do. Do the same in air... that fish can't compress the air fast enough to do anything more than wiggle in your hand. It can't propel itself at all. You can't directly compare things that exist in incompressible medium with those that exist in the opposite. __________________ "¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"