Here's a thought problem: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

I just heard about this today. Apparently, it consumed a bunch of people's time at work, resulting in a flurry of email throughout the day. But since I'm not on the "total time wasting email lists", I didn't hear about it until this evening. If you're not up to speed, a quick Google search will help: conveyor belt jet take-off problem.

Anyway, the idea is that a gigantic conveyor belt sits underneath an airplane, and you have to answer the questions of whether the plane can take off, and why.

Here's my take: people are inclined to think of the airplane's motion as they do car motion. That is, wheels are what propel a car forward, and wheel spin would cause the conveyor belt to spin, therefore keeping the car in a fixed position. But airplanes do not generate power through wheel motion. On an airplane, motion occurs as the result of thrust generated by the engines.

During take-off, wheels keep the plane off the ground so it does not drag along the runway. Wheels are not a means of propelling the plane, or applying forward motion, or anything other than keeping the plane off of the ground. That's all.

Back to the car analogy, the wheels are quite clearly related to the forward motion of the car. No wheel motion, no car movement. Everyone agrees on this. But for an airplane, you could theoretically remove the wheels altogether, and with enough engine force, drag the plane along the ground until it reached take-off speed. This is because the engine force is applied as thrust through the air, not as rotational motion applied to the wheels.

Whether the conveyor belt is there, or not, is irrelevant. Whether the conveyor belt can increase in speed is irrelevant, too.

The plane will take off.