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james
post Feb 23 2006, 01:08 PM
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Here's a question for you:
QUOTE
"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?"


A couple of links discussing it (and some funny rationalizations for peoples incorrect assumptions):
www.kottke.org/06/02/plane-conveyor-belt
www.straightdope.com/columns/060203.html


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camsmith
post Feb 23 2006, 01:21 PM
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Before I look up the links... Yes it can.


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james
post Feb 23 2006, 02:07 PM
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Anyone else?


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"We are number one, all others are number two or lower!" - The Sphinx, Mystery Men

"A computer without a Microsoft operating system is like a dog without bricks tied to its head" - annon

"What a terrible thing to have lost one's mind. Or not to have a mind at all. How true that is." - Dan Quayle

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paul
post Feb 23 2006, 03:33 PM
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My initial thought is a big no! I can explain if you like......


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james
post Feb 23 2006, 04:39 PM
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Fire away! - Show your workings out and the exam board would say.

p.s. My initial reaction was 'no', then I thought about it and concluded 'maybe - Im confused and my head hurts' and eventually 'yes'.


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"A computer without a Microsoft operating system is like a dog without bricks tied to its head" - annon

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womble
post Feb 23 2006, 04:52 PM
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erm quick change of mind after new logical suggestion.

This post has been edited by womble: Feb 23 2006, 05:15 PM


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camsmith
post Feb 23 2006, 04:57 PM
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OK... My very first thought was no, but then I thought about it and changed my mind after about 30 secs.

For a plane to move, the thrust provided (regardless of propeller or jet) comes from the surrounding air, not from traction on the ground.

Therefore, the plane moves (accelerates) relative to the air, not the ground. Therefore, regardless of the speed of the ground, the plane will start to move. The wheels on the plane will simply spin twice as fast as the plane moves to compensate for the moving ground.

The only assumptions made are that the runway is a normal length runway for the plane, and that the friction generated by the wheels spinning twice as fast as normal is negligible.


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womble
post Feb 23 2006, 05:14 PM
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hmmm yeah that actually sounds about it yeah that sounds logical


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james
post Feb 23 2006, 05:25 PM
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Yeah that's exactly it - I think what makes us all get it wrong initially is that we are used to wheeled transport - where the friction, resistance and resulting force for the wheels being powered is what gives it momentum, whereas on a plane the wheels are freewheeling (on takeoff) and are largely irrelevant in terms of the motion generated by the engines.

Good one though eh?


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"We are number one, all others are number two or lower!" - The Sphinx, Mystery Men

"A computer without a Microsoft operating system is like a dog without bricks tied to its head" - annon

"What a terrible thing to have lost one's mind. Or not to have a mind at all. How true that is." - Dan Quayle

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Sam
post Feb 23 2006, 08:12 PM
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This is Paul - Can't be arsed to log out then in again!!

My initial thought was 'what do you change in a plane to make it take off? The 'flaps' (yes I can say that).

So what is it that is effected by flaps? Air! So therefore I decided that the earth, runway, conveyor belt etc etc is irrelevant and it is the air passing under the wings that counts not the plane moving forward (although this does create the air movement in a normal take off)

Ramble over - but.................Hold on what was the question again?


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Egg Designer
post Feb 24 2006, 12:45 AM
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therefore Paul your answer is no, as the engines provide the thrust to move the plane through the air, and the wings generate the lift, on this basis, if the plane is effectively stationary due to the counter effects of the converyor belt, no air is passing around the wings an no lift is created.

plane trundles noisily along the ground. biggrin.gif


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Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers of a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae.

The rset can be a total mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

Fcuknig amzanig huh
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paul
post Feb 24 2006, 07:46 AM
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Hence my statement of ' My initial thought is a big no! I can explain if you like...... '

rolleyes.gif biggrin.gif


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Egg Designer
post Feb 24 2006, 08:36 AM
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ahh yes! will read closer next time, it was late though!


This post has been edited by Egg Designer: Feb 24 2006, 08:43 AM


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Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers of a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae.

The rset can be a total mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

Fcuknig amzanig huh
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quaker
post Feb 24 2006, 09:19 AM
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no it wont take off , paul has it almost right ( dont argue , i have a big stick ) mad.gif


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james
post Feb 24 2006, 10:14 AM
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Yes it will biggrin.gif

Think of it like this - how about the conveyer is ramped up to 1000mph (trying to move the plane backwards) - depending on how good the bearings on the wheels are they will spin like crazy and the plane will move backwards a small amount through the friction on the axles (versus the inertia of the fucking heavy plane). When you turn the engines on they push air out the of the back of themselves, causing forward motion, when the forwards motion (speed) of the plane is high enough the aerodynamic properties of the wings will give the whole caboodle lift and it will take off.
As long as the wheels are allowed to freewheel, their effect is almost irrelevant.


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"We are number one, all others are number two or lower!" - The Sphinx, Mystery Men

"A computer without a Microsoft operating system is like a dog without bricks tied to its head" - annon

"What a terrible thing to have lost one's mind. Or not to have a mind at all. How true that is." - Dan Quayle

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