Physics Puzzle

[chilluk]

Don't think this one's been posted here before - found it on a Fantasy Football forum of all places - caused a 10 page discussion!!!

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?

 

[johnny_t]

In these sort of questions, the simplest answer is usually wrong, but I'm going to go for it anyway.....

Put simplistically, the lift on a plane comes as a result of the movement of air over and under its wings. As it is effectively standing still with respect to the ground, and the surrounding air, there won't be any lift, and it won't take off.

However, at some point the runway thing will break, and the plane will whizz off forwards, or the plane will run out of fuel and be jettisoned backwards off the runway and out through the gates.

Or that's my opinion anyway.

 

[Static]

No.

 

[ninebob]

Will the plane ever take off?

I agree, no.

 

[2scoops0406]

No, as has been already stated there will be no lift.

 

[ban-all-sheds]

 

[BOB.DOLE]

zee plane vile take off verticaly as thrust vuld make no differanct to see air speed as se coveror belt acts as se thrust ans i have tried this propulsion and seems to vork vell on se sinclair c5 car.

OTTO........VINFARTER

 

[joe-90]

In a similar vein:

If a fly flies inside a van at 10mph (forward direction) and the van is travelling at 50mph. How fast is the fly flying?

joe

 

[williams89]

Avery cunning stunt.

 

[Static]

No, what will happen is the plane will apply thrust, then wheels will start to move the conveyor belt, the belt as described will accelerate. A jet planes needs about 150-200kph to lift off. If the belt is accelerates at same speed the combined friction would melt the planes tires causing the plane to nose dive into the belt and burn up in a fireball..

 

[johnny_t]

Actually, assuming there are no physical boundaries to this, the following could apply....

As the conveyor belt goes more quickly, friction between it and the air will make it drag some air along with it and creates its own wind. This is usually negligible, but at some point this 'whoosh' is going to be sufficient to make the plane take off.

And once its up in the air, the fact that its jets are pumping away and it hasn't got the runway to keep it still, means its up, up and away......

 

[chilluk]

OK it's a major clue perhaps but the key in the question relates to the fact the conveyor matches the speed of the wheels not the aircraft itself.....

 

[JohnD]

the wheels are not important in the aircraft taking off, unlike a car it does not apply traction through them. the thrust will push the aircraft along until it reaches liftoff airspeed. Ground speed is not important (c.f. aircraft carrier going into the wind).

The wheels and runway will accelerate to a very curious speed as the plane moves forward, possibly an infinite speed if such a thing were possible in a mechanical feedback loop runaway situation. Whether the tyres would skid, or burn out, or the wheel bearings, or the conveyor belt break, is an unrealistic point.

 

[Spark123]

So the answer is yes, the plane will take off.
If the conveyor belt moves in the opposite direction to the wheels (at equal but opposite speed) the only result is the wheels will spin twice as fast as they would normally. The aeroplane will still move, the lift of the aeroplane is given by the speed of the air over the top of the wing being faster than the speed of air under the wing resulting in a depression above the wing and hence lift.

 

[JohnD]

... the wheels will spin twice as fast as they would normally.

curses, I missed that.