On 29/01/2011 13:05, Gerhard Fiedler wrote:
> Without resulting average momentum at
> the bottom of the box, there is no force, and hence, I'm convinced that
> if you make the box big enough, what you'll have is a box that has the
> same weight with and without a plane flying in it, Newton and everything
> considered.

This is incorrect.
All very interesting though, I enjoyed the exchange here...

Anyway, I went back to see roughly where it started and the above looked=20
about right. Thought I'd have a quick go at changing Gerhards mind..
I don't think it matters too much about the exact mechanism of flight,=20
as this makes no difference to the bigger picture anyway (the "box"=20
feels the same force either way - essentially the atmosphere will get=20
heavier when the plane leaves the ground)
With the above statement - essentially what you are saying is that it is=20
possible to place an object (plane, heavier than air) into a closed=20
system (box, filled with air) and have it make no difference to the=20
weight. In this case, if the box is initially weighed with the plane=20
before take off - does it get lighter after the plane takes off? Where=20
does the weight go?
Will this be the same with a box filled with water and a heavier than=20
water craft keeping itself afloat?

If you take all the air inside the box, and add the plane - regardless=20
of how big the box is, or what the plane is doing, ask this question: Is=20
the total weight more than before the plane was added?
Is it possible to change a closed (and steady state) systems weight? =20
(assuming no change in gravity, nor observing from speeds close to light)






--=20
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