Question from here and here (and probably lots of other places on the net by now!).
“A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?”
Rob sent this to me this morning. I said ‘No’ the plane wouldn’t take off. I understand completely the point that the thrust from a planes engine does not act through the wheels of the plane, and I used to fly so I’m fairly sure that I understand the principles of lift/drag etc.
The issue is that the question has been specifically worded to cause to differing opinions in two types of people.
The practical thinkers will assume that the plane is using it’s engines and that the thrust will simply push it forwards, regardless of the conveyor’s speed, as the frictional force between the conveyor and the wheels is negligible and the thrust from the engines acts upon the air and not the wheels. They will say the the plane does take off, and they’d be correct.
However, the theoretical thinkers will ignore the real-world practicalities of the situation because it would never exist. Instead they abstract it to a simple object with a given speed, which is being kept motionless by something exserting a force in the opposite direction. They correctly calculate that the net speed of the object will be nothing and therefore there would be no airflow over the wings of the plane to allow it to take off. They are also correct.
In reality, both answers are equally valid depending on your point of view and how you interpret the question, which has been worded very badly (on purpose, I’m sure).
This reminds me of a question once asked by my physics teacher. If popeye is sitting in a sailboat and blows on the sail, will the boat move?