Nick is absolutely correct! An increase in pressure causes an increase in density of most fluids, as noted by Sir Robert Boyle in 1662. The thing is: air be so light that changes in speed involve only tiny pressure and density increases, of a few per cent, less than occurs during a storm front passage. So can safely be ignored and the whole wonderful century- old mathematical discipline of incompressible fluid mechanics becomes almost correct ! And true enuff!! The genius of aerodynamicers has been to convert this teeny-weeny pressure difference into a force enough to lift tons of weaponry, or i nnocent vacationers, across half the globe in fractions of a day ! It has always been an exhilarating thought to me, and gra tifying to have had some tiny part in its provenance.
A'course the birdies knew it all along, and in the case of the ravens of Sant F e, almost seem to "understand" lift!! I gav e a seminar on that at Caltech recently, and offered it to Friam, but no-one around here seems interested! I was shocked on the internet to see many " scientific" elementary discussions of the grand ol' Boyle's Law make no reference as to its date, the character of S ir Robert and how he got to that great realization . How can ya expect kids to have any sense of history when elementary science writers are such boors? And, Yes, Nicholas there are huge changes in pressure and density with altitude in the atmosphere, that fundamentally give us that popular, ubiquitous and inescapable phenomenon, the weather! It's easy to see the air near the surface of our earth is carrying, on its back, all the air above it, and so is pretty heavily laden. More, I think, tha n that turtle down at the South Pole that carries E arth on its back. Now flow speeds in the earth atmosphere (not on other planets) are low enough that compressibility due to speed doesn't change density enough to be of any importance. So terrestial meteorology fluid dynamics is incompressible but of importantly varying density. The subject of terms and usage is certainly of interest. W hen I taught this stuff, at Caltech, Stanford, USC, USN, I tried to interest students in the meaning of words, on the grounds that it was not only kultured , but also a way of understanding phenomena more profoundly. I don't think my sermons were very effective. Pity. For example, if one defines "stability" as a property where "a system with multiple states returns to the initial after a disturbance", then one finds it works for elastic, dynamic, mental, political, financial, eco logical stab. And has a vivid meaning. I think the popular usage was coined mainly by Routh in 1905 in connection with dynamic stability. Yeah, the date's correct. Them old SOBs wuz doing vehicle stability same time as the Wrights were first fluttering around! Ya gotta admire theoreticians of N atural Science. Peter Lissaman, Da Vinci Ventures Expertise is not knowing everything, but knowing what to look for. 1454 Miracerros Loop South, Santa Fe, New Mexico 87505,USA tel:(505)983-7728
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