Si el "problemilla" del avión le sigue dando vueltas en la cabeza a algunos, prefiero aclararlo desde ya y así evitar que os quite el sueño.
La solución es que el avión sí que despega, y para explicarlo hay dos teorías:
La de Blanca que dice que: "Todos los aviones despegan, porque nadie haría un avión para que no despegara". Y la de algunas personas con gafas algo más expertas en la materia que dicen lo siguiente (en ingles eso sí, pq los listos hablan todos ingles):
Elizabeth Greer says:
My father, George Springer, is a Stanford professor and past chair of the Aero/Astro (Aeronautical and Astronautical Engineering) Department, so I thought I'd send the puzzle on to the experts. My father has 50+ years of experience in structures, aeronautics and fluid dynamics, I figured it would be obvious to him. Turns out it is a poorly worded and more subtle problem than it seems at first reading (as many have commented). After a couple of days, and discussion about it with another member of the department who is THE guy - the world's foremost expert - on this kind of thing, the bottom line is that the plane will move, even if the treadmill goes backwards, and will therefore take off. Here is the explanation (with demonstration) that he sent to me:
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
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