I’ve been quite frustrated over the past day or so about the apparent stupidity of people when it comes to this problem. I read it for the first time in a thread on a forum I frequent – I read this ‘riddle’, and was astonished that so many people could not understand the blindingly-obvious solution.
The riddle is:
A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, whilst the belt moves in the opposite direction. This belt has a control system that tracks the plane’s speed and adjust the speed of the belt to be exactly the same (but acting in the opposite direction). Can the plane take off?
Here’s where a good proportion of you jump right in and go “No, of course not. Speed + (-Speed) = 0″. And that is where you fail. Let’s see why…
The plane’s actual speed (not wheel speed) is the speed with which it moves in reference to a fixed point – this is the only speed that matters (i.e. the speed at which the plane moves). This is obvious when we understand that the plane is not accelerated through the force of friction against the ground, applied by the wheels (as, for example, it is in a car), but rather through the force exerted on the air by the jets/props (a plane’s wheels are simply there to keep it off the ground, their rate and direction of roll are of no consequence).
Given the plane can still be accelerated by its engines and is still supported on its wheels, the plane will accelerate as normal and, therefore, take off as normal.
(Of course, this entire post assumes various things, such as that the wheels can survive the speeds at which they will rotate, etc, etc. – but the principle still stands.)