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of course there is speed built up - it's just the imagination that's tricked...
an airplane's wheel is like a skate, there's no gear as in a car that 'connects' it with the ground (belt in this case)
the latter just slips through under the wheels while the engine 'pulls' forward

cheers, Tom

yet there's no speed built up relative to the air, so what airstreem could be responsable for a lift up?
there is no airstream other than the air that get's sucked into the engine's housing.
the built up power by the engines just leaks away thrue the turning wheels on the conveyer, or?

lets try it another way...
there are bearings in the wheels of the plane which stands on the conveyor belt
agreed ? bearings are constructed in a certain way (and are lubricated) to reduce friction as much as possible

now assume these were ideal bearings with no friction at all...
if the conveyer belt would be started, the plane simply wouldn't move - it's weight would work 'against' zero friction in the wheels' bearings

with the belt moving, the tyres of the plane would start to rotate around their axis (due to friction between rubber and belt), but they wouldn't move the rest of the plane because there's no friction to transmit the momentum (in those ideal bearings)

in reality the backward running belt would require the plane's engine to compensate for the force generated by the plane's weight against the wheels' friction to reach liftoff speed, but it would in no way stop it

cheers, Tom

thanks for another try
But it doesn't answer the crucial question what airstream is available to lift the plane...there hardly is any airstream over the wings (to cause underpressure at the upperside of them)
Just like on a home trainer, my hair won't wave (lift off, so to speak ), as you mentioned already, if I'm right it was you.

(I'm familiar with the concept of experiment of thought, so practical disturbancies from the ideal situation like friction don't confuse me).

Bytheway, how would a lift-off from the belt look like?
Hanging in the air for a second, then shooting away like a rocket?
Or slowly going high, starting at speed zero (relative to the hard ground) up to 600 miles an hour?

I don't know, just asking questions

hubird wrote: Bytheway, how would a lift-off from the belt look like?

that easy?

but not much speed...

hubird, if the plane would not catch speed, the conveyor wouldn't start to roll either. read the riddle thoroughly

hubird wrote:that easy?

Those are marijuana leaves if no one noticed

This question turned our office into a Christmas pantomime last Friday, with cries of "Oh yes it will!" and "Oh no it won't!" echoing abound the building

I must confess to having changed my own mind about three times whilst thinking this through, but I think that the plane can take off - the treadmill will only cause the rotational speed of the wheels to increase; it will not prevent forward motion of the plane (note - the wheels are not driven and so can effectively freewheel).

of course, the plane will just make it's usual speed, the wheels will roll faster, and airflow is built up
Now I'm gonne think over why I was thinking that the conveyer keeps the plane on it's place...
thanks Royston, and others who didn't get me so far

Naturally, thinking about this problem lead me to another:

Imagine you have an inclined, flat treadmill. If a ball is placed on this (e.g. a football), then the ball rolls down the hill (as you would expect). (Note: treadmill is stationary in this example).

Is it possible to modify the speed of the treadmill to prevent the ball from rolling down?

Crikey.....

No the plane will not take off. The wings must move relative to the air, to provide lift. This will not happen as the treadmill keeps the aircraft stationery relative to the air. The plane will sit with engines screaming and wheels spinning.

emzee, why will the wheels spin if the plane's not moving? the wheels are not powered, they move because the plane moves. the reason the plane won't take off is because the bearings fail from spinning so fast(friction).

well, a standard 61806 bearing (greased) is speced for 15k rpm which is eqivalent to an effective speed of 900 km/h if the diameter of the tyre is about 0.35m (to keep the calculation simple...), with a dynamic load of 4.5 tons per wheel...
probably the rubber will dissolve first

cheers, Tom

that sounds about right...

emzee wrote: the treadmill keeps the aircraft stationery relative to the air.

that's exactly were i got confused...as Garyb and others said, the wheels just will turn faster to the degree the conveyer dictates -not accounting the melting rubber

well then - plane eventually nosedives into belt and some parts take off backwards

If the belt motor packed in suddenly, the plane would shoot forward & straight through the passenger terminal before take-off

true, if the pilot forgot to loosen the brakes