I’m surprised this hasn’t been covered here on Gadling. Here’s the hypothetical situation:
If a standard airport runway were replaced with a large conveyor belt — one moving in the opposite direction (and equal speed) that a plane intended to take off in — would the plane ever leave the ground?
Your first thought is no, it won’t take off — I thought this too at first. A car in this situation, for instance, wouldn’t move forward or backwards at all, as long as the conveyor belt was moving in the opposite direction at the exact same speed as the car. If the belt speed and car speed were mismatched at all, the car would move forward or backward. This is because the engine rotates the wheels over the road, and relies on the friction between the two to move the car forward. But a plane is different, and here’s why:
A plane doesn’t rely on friction between its wheels and the ground to take off like a car does. On take-off, the wheels act as a sort of lubricant between the plane and the ground; there is no motor turning them forward to progress the plane, they spin freely. They are only there to make the plane roll down the runway easier. After all, without wheels, the plane’s belly would screech across the ground and cause a lot of friction — but it would still take off, assuming the jet engines are powerful enough to makeup for the added friction. What propels the plane forward are its jet engines, not the wheels; it doesn’t matter if the plane is on a concrete, ice, water, grass, or a conveyor belt — the only thing an opposite-moving conveyor belt would do to the plane is cause its wheels to spin much faster than if it were on a regular, concrete runway. The plane would still take off. This was a hard concept to grasp, but once I did, it made sense.
What do you think?
P.S. This was covered in the NY Times a few months ago, and there’s a HUGE discussion on the true answer to this conundrum; there’s almost 900 comments on David Pogue’s NY Times blog, with conflicting opinions from laymen and physicists alike. Have a look, but be careful — it takes a while to load and may crash your browser due to the number of comments.
Photo by j_wijnands