illuminating science

If you’ve never heard this question, you’re in for an experience. This (along with the Monty Hall problem) has got to be the most controversial problem in physics, ever. Seriously. Check out these discussions. It gets seriously aggressive!

So what gets people so impassioned? Here’s the problem:

A plane is standing on a runway that can move (some sort of conveyor belt). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?

Or, a better phrasing is this:

A plane is standing on a runway that can move. The plane moves in one direction, while the conveyor moves in the opposite direction at the plane’s take off speed. Can the plane take off?

Stop and think about it. Then, read on

My first answer was…no. My (incorrect!) thinking was that it’s the speed of the air over the plane wings that provides the lift for a plane to fly. Hence if you were on a backwards moving conveyor belt, so that you weren’t moving relative to the air, there would never be any lift no matter how fast you went and you could never take off.

But I “misread” the question - I’m right in part - we’ll all agree that if a plane is not moving relative to the air, then it cannot take off. (It’s the motion of air over the wings that gives it lift.) But what this question is asking is really this: Can a conveyor belt stop a plane from moving and accelerating forwards through the air? Or, another way, can a plane on a conveyor belt move forward through the air, no matter how fast the belt is moving?

The answer is yes - it can and will.

Let’s explain this three times -first, with facts, second with physics, then finally building up a logical argument with more details. I’m using ideas from a variety of sources, including Straight Dope, doesn’t depend on speed, for all intents and purposes.)

So, finally, to our plane. The wheels on a plane are only used for rolling or braking - they’re not “powered” in anyway, just like a rollerblade. Instead, a plane is able to accelerate quite independently of the ground (e.g., in mid air!), by using its engines or a propeller. No matter how fast the conveyor belt is moving, I can use a small amount of my thrust to overcome the friction there (just like the roller blades), and the rest goes into accelerating the plane. That means that the plane can and will start to accelerate forwards, eventually reaching take off speed. Again, in terms of forces, the only forces acting (horizontally) on the plane are the thrust from the engines and a tiny bit of friction from the imperfect wheels. It must move forward, by the laws of physics, and so will take off.

Just remember, that people who say a stationary plane can’t take off are absolutely right! But that’s not the key to this question - it’s whether a moving conveyor belt can stop a plane. And, it turns out, it can’t. This is really tough to get your head around at least in part because the problem is badly worded. Both sides have valid points, you’ve just got to agree on the problem. (In particular, the phrasing that the “belt moves at the same but opposite speed as the plane” - as the plane compared to what? Bad question!)

And the final words, of course, comes from Mythbusters who showed the plane can take off. It’s not a perfect experiment, but the basic idea is correct and shows the correct physics. (A moving tarp is a pretty good approximation to a conveyor belt - even if you can feel the ground through it, that shouldn’t change the friction much! The ground is still “moving”.)

I hope that helps clear things up!