Sunday, April 9, 2006

Up, Up and Away!

Here’s a question for you that really bugs me:

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?

This question has been haunting the internet for some time now, but I didn’t see it until yesterday. I’d say it’s impossible for the plane to take off, but the great minds of the internet says that it will take off as a normal plane regardless of the conveyer belt. They’re also claiming that you’ll only need Physics 101 to solve the exercise…

Is this just case of human intuition being plain¹ wrong - like the classic “do you swim slower in syrup than in water” - or have the great minds of the internet teamed up to confuse the rest of us?

Can someone explain this one to me? Preferably in a language that’s on my intellectual level. Baby language, perhaps.

The pun, oh, the pun. ↩︎

Feedback

Do you have any thoughts you want to share? A question, maybe? Or is something in this post just plainly wrong? Then please send an e-mail to vegard at vegard dot net with your input. You can also use any of the other points of contact listed on the About page.

Klas

2006-04-09 19:32:45 GMT

the reason why a plane takes off in the first place has to do with the shape of the wings. the upper side is more curved and thus the air travels longer to reach the other side. that creates a low pressure on the upper side of the wing and will lift the whole plane.

I would therefore be so bold to say that the airplane on the conveyor belt would not lift off because it is stationary compared to the air around it.

Roar

2006-04-09 22:43:06 GMT

Well, since a plane doesnt use the wheels for propulsion it would move almost as if the plane was running on a normal runway. The only thing would be that the wheels will spin twice as fast.

Øyvind

2006-04-10 11:09:33 GMT

I agree 100% with Roar. The amount of trust from the engines have to be increased some though, due to friction from the wheels.

Roar

2006-04-10 11:58:30 GMT

I see that some people on the great wide internet use the friction the wheels will produce to counteract the force the engines produce. In that scenario the plane would stay put, but the question posted is that the conveyor belt would try to move in the same speed as the plane itself. So if the plane on a normal surface would move 20kmph the conveyor belt would move 20kmph the oposite direction, resulting in the wheels of the plane spinning as if the plane where going 40kmph on a normal surface. That would not be enough friction to keep it standing still.

Øyvind

2006-04-10 12:21:18 GMT

Hmm, I just thought of this, if the plane doesnt move because of the force the conveyer applies on the plane.. the conveyer cant move either. Because it clearly states that the conveyer moves at the same speed as the plane. So the final solution is this: the pilot has gone home and the conveyer is broken.

abbagold

2006-04-12 12:18:25 GMT

i totally agree with Klas..
the plane is moving on the ground, but the air would be still around it’s wings. If you were to use a bicycle on a conveyor belt, you would not feel the "wind in your hair" now would you?

Explanation in Baby Language as requested

2006-04-18 14:57:48 GMT

Ga ga guuu.. Blagaa!! Guuu, ga ba ba BA!! Guuuuh bla gaga bububububbu.. Weeeeee!!!!

Vegard Skjefstad

2006-04-18 20:37:40 GMT (post author)

Of course! It all makes perfect sense now!

← Bill Follow Up, Up And Away →

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