Brainteaser Options

[james]

Here's a question for you:

"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?"

A couple of links discussing it (and some funny rationalizations for peoples incorrect assumptions):
www.kottke.org/06/02/plane-conveyor-belt
www.straightdope.com/columns/060203.html

[camsmith]

It's all about relative speeds...

A car's speed is measured relative to the ground immediately below its wheels.

A plane's speed is measured relative to the air around it.

I agree that if you put a standard motor car in the place of the plane, it would absolutely not move. This is because the car can only move if its tyres provide friction with the ground. As the wheels go round, the car moves across the ground.

However, we are talking about a plane. The plane moves because it forces a lot of air backwards. The mass of air being forced backwards exerts the same force on the plane to move forwards through the air.

The fact is that normally, the air and the ground normally move at roughly the same speed and therefore we perceive that the same thing is happening. It is not. The plane is moving through the air, the car is moving across the ground.

If we start moving the ground, the car will be affected, but the plane will not.

This is exactly the same as saying that if a wind blows, the plane will be affected, but the car will not. This is why planes always take-off and land into the wind.