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Old 05-02-2006, 06:17 AM   #77
DaveS
Join Date: Nov 2005
Posts: 91
United Kingdom
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Re: The "Jo Trick" and Similar Exercises

Quote:
Paul Nowicki wrote:
I'm not sure why planes are such a mystery here. Its perfectly explainable and quite basic why planes fly. Of course the reason is the airfoil of the wing generates the air on top to travel faster than the air on the bottom creating low pressure on top of the wing and high pressure on the bottom. This pressure differential pushes the wing up, therefore the plane goes up. What is so mystical about that? And what is inaccurate about gravity or Newton's laws? Am I missing something here?.
Going noticeably off topic here, but to quickly explain myself...

Newtonian gravity isn't a completely accurate picture of the universe - for instance, the orbit of Venus (iirc: it might be Mercury) has slight discrepancies with what Newton's laws predict - it accelerates at a slightly different rate and in a slightly different direction from what you'd expect. General relativity describes the motion of the planets far more accurately. Thus GR is a more accurate and more 'correct' theory, and Newtonian gravity, having been falsified experimentally, is 'wrong' - from the point of view of a theoretical physicist, it has no more relevance than the flat earth theory.

However:

GR is a very complicated theory and Newtonian gravity isn't. And it's accurate enough in limited situations, including most of those that we encounter in (say) engineering. Thus people use it to design planes. Using general relativity to do the calculations to design a plane that stays up would be highly impractical, although it'd almost certainly lead to the same conclusion. The explanation in terms of air pressure and and gravity is a bit of a fudge, but it's accurate enough that it always works in practice.
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