RichR
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copying and pasting comeone else proves the point - you will believe anything you read
Everything in that statement has been discussed and IMO dismissed
and the final few lines - you cant simply ignore the conveyor - its there - the whole question relates to the use of a conveyor - you cant ignore it; it is assumed to be a physical entity and it does have an effect on the plane
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Steve
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Registered: 30th Mar 02
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read my post just, that should (hopefully) settle it
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Steve
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quote:
Originally posted by LiVe LeE
copying and pasting comeone else proves the point - you will believe anything you read
Everything in that statement has been discussed and IMO dismissed
and the final few lines - you cant simply ignore the conveyor - its there - the whole question relates to the use of a conveyor - you cant ignore it; it is assumed to be a physical entity and it does have an effect on the plane
this is the key bit, this is fact, and even you know its fact
"The wheels of an aircraft are "free-wheeling". They do not provide propulsion, and therefore do not "push against" the action of the conveyor belt.
The thrust for aircraft movement comes from jet engines or propellors... not the wheels. Therefore, the thrust being applied to the aircraft body is completely decoupled from how fast the wheels happen to be spinning.
Thrust acts according to Newtons Third Law of Motion - every action has an equal and opposite reaction. The thrust of the engines is acting against the air.
Because the wheels are free-wheeling and we have assumed zero friction at the hub, it follows that the conveyor belt, no matter how fast it is moving, CANNOT EXERT ANY FORCE on the aircraft with respect to forward motion! There is no force in our experiment that can oppose the thrust vector of the aircraft."
[Edited on 24-05-2006 by Steve]
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RichR
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Location: Waterhouses, Staffordshire
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[qoute]
X = Wheel Rotational Speed
X = Conveyor Belt Speed, as per your conditions
Z = Speed of Aircraft = Some non-zero positive number
The equation is: X = X + Z, which is illogical
yes because its not true
W=X=Y is the correct equation!
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Jules S
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Registered: 24th Dec 03
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You most certainly can ignore the conveyor belt.
It has no effect whatsoever on the planes ability to move forwards.
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Steve
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what that guy said is true, say there is 100lbs of thrust pushing ->
there is not 100lbs of force being applied to the aircraft <- by the conveyor
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RichR
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Location: Waterhouses, Staffordshire
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they can exert force though - the friction component is a force acting - it is this which must be overcome in order to increase the velocity of advance! - this wont be overcome given the conditions implied by the question
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RichR
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when the planes on the ground why does it have wheels?
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Cosmo
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Steve
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yes friction, but the friction force would not be the same or greater than thrust force, it woul dbe very small in relative terms
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RichR
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and why do the wheels have braking systems?
if they have no relation to the speed of the aircraft ffs
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Cosmo
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quote:
Originally posted by Steve
yes friction, but the friction force would not be the same or greater than thrust force, it woul dbe very small in relative terms
like Ive said before..have you dont the maths or just saying this? As I could just say the frictional force will be greater than that of the jet engines.
So once again...pointless arguing unless someone works out the maths on this...if you want the real answer anyway!
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Steve
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because then you are placing a "lock" or great frictional force between the aircraft the wheels and the ground, if you dont apply the brakes there is no force between plane wheels and ground, or very little anyway
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Mase
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Registered: 16th Sep 01
Location: Derbyshire
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yes, but i believe that the thrust provided by a commerial jet engine would quite easily be able to over come these frictional forces ??? as normally there are only 3 contact points between the plane and the ground and the wheels are not that large in the general scheme of things, therefore the contact area at any one time isn't that large.....
[Edited on 24-05-2006 by mase5]
Mase
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Jules S
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Registered: 24th Dec 03
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Funny,
I dont think anybody has mentioned landing on this mythical runway.
if what Lee is saying is true the plane would stop dead as soon as it touched the ground.
That sounds likely doesn't it?
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3CorsaMeal
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Registered: 11th Apr 02
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all i know is
canderel in the morning
canderel in the evening
canderel at supper time
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3CorsaMeal
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Registered: 11th Apr 02
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quote:
Originally posted by Jules S
Funny,
I dont think anybody has mentioned landing on this mythical runway.
if what Lee is saying is true the plane would stop dead as soon as it touched the ground.
That sounds likely doesn't it?
genius 
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Steve
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yes jules perfect mate, if you land on it you will still roll forwards with momentum the same as normal if you didnt apply the brakes
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Cosmo
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quote:
Originally posted by Steve
yes jules perfect mate, if you land on it you will still roll forwards with momentum the same as normal if you didnt apply the brakes
or look at it the other way round. If the wheels free wheel then you can land the opposite way but not go lying off the end 
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Steve
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yep, are you getting it now cosmo
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Cosmo
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quote:
Originally posted by Steve
yep, are you getting it now cosmo
but you would 
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Ojc
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If I had a sausage, and Steve wanted half and Cosmo wanted half, but Jr wanted all of it, how could I satisfy everyone with my sauasage?
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Cosmo
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quote:
Originally posted by Ojc
If I had a sausage, and Steve wanted half and Cosmo wanted half, but Jr wanted all of it, how could I satisfy everyone with my sauasage?
can we take it in turns 
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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quote:
Originally posted by Jules S
Funny,
I dont think anybody has mentioned landing on this mythical runway.
if what Lee is saying is true the plane would stop dead as soon as it touched the ground.
That sounds likely doesn't it?
only if its speed was identical to the conveyor at the time of landing - and seeing as were assuming that the conveyor is driven by the plane, then no it wouldnt!
however it would slow the plane considerably faster than airbrakes/wheel brakes alone
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Paul_J
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Registered: 6th Jun 02
Location: London
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quote:
Originally posted by Steve
because then you are placing a "lock" or great frictional force between the aircraft the wheels and the ground, if you dont apply the brakes there is no force between plane wheels and ground, or very little anyway
Jesus christ, are people still not understanding steve?
What is up with the people on this forum - Where does it say planes propel from the wheels and WHERE does it say anywhere in the question that the plane is unable to move.
It says the convey moves in the opposite direction at same speed as moving plane.
So Plane has already started moving forward say 1 ms (which breaks the hardest part of any movement, the initial move off is where things are at highest load, hence 1st gears on cars are short etc) - the conveyor is moving backwards at 1ms (so the conveyor is only moving very slowly) - the engines are at full thrust pushing one way. The only resistance going backwards is the resistance of the wheel bearings etc.
The wheels will free roll (below > > << show forces)
at start _____ <<<< PLANE ____
after initial movement ____ <<<< PLANE ____ >
still moving ___ <<<< PLANE ______ >>
The planes thrust @ maximum will stay at maximum thrust from the start (well once the engines have spun up) - the force backwards regardless of the wheel bearings resistance will be very low as the conveyor will be moving very slowly at the start. The plane will accelerate, the conveyor will not produce a 'equal' force in return to the planes thrust as the plane's wheels are free spinning, not locked. The plane will take off, but it may take a little bit longer than a normal run way to do it due to slight restistance against it.
Plane wheels are in no way connected to the plane, they just roll, hence they need to be towed out - its then the planes engines that thrust and move them about the air field, not the wheels.
Steve's initial example was great.
Picture a toy car, with no engine, gears or anything. Just wheels on little axels that spin freely. You place the car on a conveyor belt while your mate increases the speed slowly. the force required to hold the car in the position is only relative to the resistance through the wheel bearings, as essentially the wheels will be free rolling and all energy from the conveyor onto the tyres are being used to spin the wheels.
You would not have to push the toy car at the equal force as the conveyor belt to hold it in place. If you did, then the car would have locked wheels and be sticking in the conveyor.
Basically I can understand people mis-reading the question in the 1st place. but ASSUMING that the conveyor belt just goes backwards at the SPEED of the plane going forwards - then the plane will take off.
TOO MANY people are thinking it says The conveyor belt will impose a EQUAL FORCE as the plane going forward - which is not the case!
The forward on the plane to combat the thrust will be the FORCE OF conveyor going backwards divided by the amount the 'resistance of the wheel bearings' will have.
Since even if the wheels were locked the thrust would probably drag the plane along the ground at low speeds, I can't see how people can't picture that the plane with free wheeling wheels could move forward on the runway and take off.
FFS.
Steve is right.
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