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Inspiré de "Une scytale informatique" par J.-P. Delahaye et P. Mathieu, Pour La Science, Septembre 2007.
A blog about science, ideas, and whatever crosses my mind
Il m'est parvenu, qu'au temps du Califat Al Mamoun, vivait à Bagdad un vieil homme malicieux. Quand on venait lui annoncer le mariage prochain de la fille d'un de ses amis, il demandait invariablement:"Contre qui se marie-t-elle?".
La femme du vieil homme venait alors s'asseoir tout contre lui, et vraiment leurs yeux pétillaient.
Mais Allah est plus savant encore.
Extrait d’un article signé Ron Suskind, intituté « Faith, Certainty and the Presidency of George W. Bush », paru dans le New York Time du 17 octobre 2002.
J’imagine qu’à l’époque la Maison Blanche n’imaginait pas que la réalité la rattraperait un jour à si grand pas, comme elle semble le faire en Irak.In the summer of 2002, after I had written an article in Esquire that the White House didn't like about Bush's former communications director, Karen Hughes, I had a meeting with a senior adviser to Bush. He expressed the White House's displeasure, and then he told me something that at the time I didn't fully comprehend -- but which I now believe gets to the very heart of the Bush presidency.
The aide said that guys like me were ''in what we call the reality-based community,'' which he defined as people who ''believe that solutions emerge from your judicious study of discernible reality.'' I nodded and murmured something about enlightenment principles and empiricism. He cut me off. ''That's not the way the world really works anymore,'' he continued. ''We're an empire now, and when we act, we create our own reality. And while you're studying that reality -- judiciously, as you will -- we'll act again, creating other new realities, which you can study too, and that's how things will sort out. We're history's actors . . . and you, all of you, will be left to just study what we do.''
A quand le mythe Platonicien de la caverne comme idéal de réalisme politique?
On the first picture I am hiding behind the cabin and pumping. Less than 0.2 seconds later, the bottle has expelled all its water and it is 5 meters high. With some rough kinematics (velocity = acceleration*time, distance = 1/2 acceleration*time squared), one finds that the average acceleration of the bottle is more than 25 g’s, and its velocity after 0.2 s is more than 50 m/s!
The inset shows the nicely even bouncing of the water jet on the ground at the rocket’s takeoff. The cloud in the second picture (indicated by an arrow) is formed when the bottle empties entirely from all its water. From the second and third pictures, it is interesting to note that, although the water is being expelled downwards, the water of the cloud is moving upwards. The reason for this is that the water is expelled downwards relative to the bottle. As the bottle itself is climbing faster than the water is descending compared to it, the absolute velocity of the water is still upwards.
Incidentally, this is what multistage rocket systems aim at avoiding [3]. The first stage uses low exhaust velocity fuels, and high exhaust velocity fuels are used in the upper stages. Under these conditions, the exhaust velocity increases when the rocket gains speed: the fuel is always expelled backwards, which makes the propulsion very efficient.
It would be interesting to check whether the same can be obtained with a water rocket by optimizing the shape of the bottle.