Physics Puzzle

[Softus]

Don't think this one's been posted here before.

An aircraft is standing on a very long runway that can move (a conveyor belt). The aircraft moves in one direction, while the conveyor belt moves in the opposite direction. This conveyor belt has a control system that tracks the aircraft's wheels' speed and tunes the speed of the conveyor belt to be exactly the same as the wheels, but in the opposite direction. There is no wind. The pilot begins to add thrust to the engines...

Will the plane ever take off?

 

[WDIK]

no because the plane is effectively standing still.
for the plane to leave the ground there has to be a negative draft above the wing to create lift, as no air is passing over the wing negative draft cannot be acheived

 

[Softus]

But surely the plane will move forward under power of its engines?

 

[xerxes]

OK, Ill have a go at this.

As the landing gear wheels are freewheeling, the conveyor cant apply much rearward motion to the plane (apart from what results as a consequence of friction in the bearings etc.)

So, no matter how powerfully the conveyor is driven, the thrust of the planes engines wins.

My answer, then, is that the plane takes off.

 

[Softus]

Hm. But as the conveyor matches the aircraft speed, surely it can never take off?

 

[WDIK]

But surely the plane will move forward under power of its engines?

thats why it stands still other wise it would be off the belt

 

[WDIK]

it stands still and because it does not actually proceed in its static space neither does the air therefore no lift over the wings

 

[Softus]

Good point. But if moves off the belt then wouldn't it take off?

 

[WDIK]

Good point. But if moves off the belt then wouldnt it take off?

no it would crash in reverse

 

[xerxes]

The conveyor could go twice as fast as the plane, and the plane would still be able to take off. The speed of the conveyor is irrelevant to its ability to counteract the thrust of the engines.

 

[WDIK]

unless the wings were fitted backwards

then so would the engines have to be

 

[WDIK]

The conveyor could go twice as fast as the plane, and the plane would still be able to take off. The speed of the conveyor is irrelevant to its ability to counteract the thrust of the engines.

if that were the case the plane would move in reverse while it is physically related to the conveyer the thats the motional influence

 

[xerxes]

Are the brakes off? And the chocks removed?

 

[WDIK]

Are the brakes off? And the chocks removed?

that was your job not mine

 

[xerxes]

I didnt start this.