Hot Air, and Compressibility
A’course air is compressible, and so is water. Any kid who fools around with bicycle pump or a shock absorber, and believes what he sees, rather than what he is told, can feel the compressibility. But, but, but, at modest speeds free air will NOT compress and prefers to “run away” rather than doing so. For example, if you sweep your hand through the air, the flow runs away and, although the pressure on your palm is higher than static, there is virtually no compression of the fluid. You have to sweep your hand at a speed comparable to that of sound (about 330 m/s here on earth) in order stop the air from getting away and to achieve any compression. Since, according to my latest studies on musculature, the maximum speed of an Olympic discus hurler is about 33 m/s, we’re not likely to experience that. Anyhow, the effective incompressibility of air is taught Day 1 in Aerodynamics 101! The reason WHY is reserved for four years later in grad school!
Surely anyone who can read today has heard of the “sound barrier” or seen the movie, if you can’t read. Many folks in the West have experienced a sonic boom. All this has been known since 1890 and was described, analyzed and measured by Herr Doktor Ernst Mach, a product of that golden milieu of scientific thinkers, the old Austro-Hungarian Empire. Mach was a rare bird, being an intelligent philosopher who discussed physics with Einstein. He didn’t agree with Special Relativity. Wrong there, Ernie! Some people have heard of his number – certainly all test pilots. I instructed many in the Navy. I painfully learned this stuff, and used the instrument Mach invented, the Schlieren, in my early days as a grad student, battling with shock waves in a grimy subterranean Wizard’s Cave, the Hypersonics Lab, not at, but underneath Caltech!
All the above info I stand behind, and have published on. It’s not conjecture or anecdote. I don’t discuss things I don’t understand. Like most scientists, I am an ignorant fellow, oblivious to the vast majority of human knowledge, but enjoy being enlightened by folks who do know. It is not dumb not to know things, but it is to think that your own knowledge encompasses physical truth on subjects on which you are ignorant.
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