Brain Teaser

Thanks to Andrew W for the link.

Here's a "gotcha!" scenario to test the minds of the scientists and engineers out there. I'll put the solution in invisible text following the question:

"On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

Answer:
The plane can take off.

The problem is that many people are thinking of the moving conveyor like we would a treadmill - when we run on it, we are exerting forward momentum/force by acting on the treadmill, using friction to push ourselves along the ground. Thus, to an impartial observer on stable ground, a runner on a treadmill would be going nowhere, simply running in place. If a plane was actually just rolling in place and not actually moving to the point that air flowed over the wings in order to generate lift, it would not be able to take off.

However, a plane is not acting on the ground - it is acting on the air. The propeller or jet engine exerts a force upon the air that the plane is in. The wheels are free-spinning and do nothing except hold the plane off the ground. Because the wheels only offer a minimum of resistance, the conveyor belt will not impede the forward motion of the plane because the plane is not relying on a driving force from the wheels to move.

So in this scenario, to an impartial observer not on the conveyor belt, the plane will move down the "runway" exactly the same as on a stable runway, except that the wheels are spinning twice as fast (due to plane's forward momentum and backward speed of conveyor belt). Once the plane achieves lift velocity, it will take off.

Kinda neat exercise. I'll admit that I didn't get it the first time I thought about it.

For more details, see this article.