A Riddle To Do Your Head In...

[SchumieFan]

if a plane could take off on a treadmill dont u think theyd just build a whole lot of tread mills rather than getting a resource concent to build a massive f-ing runway??? cmon guys forget the greys... yeah blondes r showing through!

[Supplanter]

The treadmill/runway would still need to be the conventional length for the plane to get up to enough speed to get enough lift on the wings.

[ahew]

The engines suck air through them and push the air out at a faster rate to make the plane move, they do not drive the wheels.

yep i realise that but you still require friction to move any object and give it momentum

Stick a fire cracker on a matchbox car, borrow your Dad's belt sander and do the experiment yourself

yeah but the belt sander won't be moving at the same speed as the matchbox car, even when it's acceleration increases. it not the way to test.

a closest test would be to put a car on a chassis dyno with the wheel hubs stuck on the dyno (car is motionless ie. no friction), stick a jet engine on it and see if it moves. according to this theory, the car will be able to move forward due to thrust. don't think it would....

[Chiangstar]

dont you think we're looking at this too literally for a riddle? we're not really thinking outside the box

simon

[Supplanter]

The engines suck air through them and push the air out at a faster rate to make the plane move, they do not drive the wheels.

yep i realise that but you still require friction to move any object and give it momentum

Then how does a rocket move in the vaccuum of space? No friction there Thrust goes one way, rocket goes the other. The thrust doesn't have anything to push against, other than the rocket itself.

Newton's Third Law of Motion:

For every action there is an equal and opposite reaction

[scarecrow]

So in summary, myself and Supplanter win!

The plane would take off. The operation on the conveyor belt has a negligable effect on the planes movement/momentum, because the plane's body is not connected to the conveyor, just the free spinning wheels. Sure the movement of the wheels would have some retardation on the planes forward motion, but once the jet engines started to propell it forward at decent speed it would be as if it was just taking off on any normal runway.

[akuma3]

well can we can get answer now Slither?

[SchumieFan]

The engines suck air through them and push the air out at a faster rate to make the plane move, they do not drive the wheels.

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yep i realise that but you still require friction to move any object and give it momentum

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Then how does a rocket move in the vaccuum of space? No friction there Thrust goes one way, rocket goes the other. The thrust doesn't have anything to push against, other than the rocket itself.

Newton's Third Law of Motion:

For every action there is an equal and opposite reaction

i didnt say that... there is friction in space though... hence the big bang... theres just no air

[Supplanter]

i didnt say that... there is friction in space though... hence the big bang... theres just no air

Friction is when one object rubs against another... space is a VACCUUM, it is EMPTY, so there is nothing for the controlled explosion coming out of the rocket to push against, no rubbing.

Anywho I ain't saying anything else.
Please put us out of our misery, Slither.

Maybe we should ring up Dr. Karl next Thursday morning and ask him ourselves

[scarecrow]

Sorry Schumie, but as Supp said, there's no friction in space. The Big Bang occured as a result of such an immense amount of matter collapsing into a single point (a singularity) that it simply became too dense and too hot to exist as that singularity, exploding outwards with great force.

There was intermolecular friction inside the singularity, but there is certainly no air friction in space.

Spaceships move by expelling gas from burnt fuel. They can only carry as much momentum as the gas creates when being pushed out of the craft.

Generally friction retards movement and momentum (well, in all cases I can think of).

[pagan]

hmm, personally i dont care if the plane takes off or not.... i say wait for the plane and convayer belt to hit max speed then reverse the convayer belt to forward motion.... HOOOWEEEEEEEEEEEEEEEEE!

place your bets...

1: Does the plane peel like a banana BEFORE: the later can happen

A: The pilots eyes end up in his ball sack
B: The stuardess' all melt into one, veery leggy, very busty looking mess
or C: The plane becomes empty barring the mass of seats and pancake flesh and blood seeping into the tail
D: plane finds itself in another dimension where someone can guess fly or not fly and be right, becasue here all your wishes come true
E: All the above

or 2: Do any of the 1: ABCD above happen BEFORE: the later can happen

A: The wings peel off, turning the plane into a torpedo
B: Plane front and rear instantly become one a moment before the worlds views a wicked although deadly confetty shower.
or C: option A of number 1 takes place and plane dives immediatly into the ground, saving everyones family the burdan of burial.

good luck while we wait for the all amusing answer of..... yes or no

[Bennoz]

Im getting some pretty wack mental images of a 1B & 2B.

mmmm busty confetty....

[ahew]

hmm... how did we get from an earthbound example to space? "what the?". space has weightlessness, no air, blah blah blah. completely different from a dry land example.

so.... by that theory if a person was trying to get themselves off that conveyor belt all they would have to do was throw an object in the opposite direction, creating an opposite reaction which would then magically see him off the conveyor belt. "i no think that work mista".

anyway.. no more! this is an absolute joke, where's the answer?

[scarecrow]

Google is your friend.

From Newton's 2nd law: E(sigma) F=ma

or the EFx+y (vector some of all external forces acting on an object)=mass(*)acceleration.

In our example there are two opposing forces (we shall ignore lift and weight since it seems we all agree that with the proper airspeed, the plane will fly)

These forces are the thrust of the engines (x)and the friction of the wheel system (y) as it interacts with the moving conveyor system.

Friction is represented byFy= fk=ukn where uk is the coefficient of kinetic friction, which for steel on steel ball bearings is .17, and that is not allowing for the friction reducing effects of synthetic grease on the ball bearings.

Thrust is represented byFx= F=ma where m=the mass of air being processed through the engines, and a=the acceleration of that mass due to the burning of the jet fuel, increases in temperature and volume of the air, and the vector effects of the nozzle and fan design.

If Fx=Fy the combined but independent systems are at equilibrium and the plane has zero airspeed regardless of wheel speed. If Fx>Fy then a will be positive assuming m stays constant, generating positive acceleration, airspeed, and liftoff. If Fx<Fy then the engines will produce thrust, but will not be able to overcome the Fy and the plane will have zero airspeed and zero wheel speed. some the forces and you have your answer. Its not really that important if the airplane will fly or not, what is important is our understanding of the physics leading to the correct answer to the question: Why?

It is important to note that we are dealing with two iindependent systems here, the atmospheric system and its relation to the engine thrust, and the ground system and its relation to the friction of the wheel system.

It is my opinion (and that of the USAF, and other pilots in the airline I now fly for) that Fx>Fy and that the airplane will be able to fly, with the caveat that the wheel speed will be twice as fast as the ground speed of the airplane. We are ignoring tire speed limitations and bearing limitations for the purpose of this thought problem.

No official answer was ever given with the question, but this is probably the best one in the 14pages of debate.

In summary, myself and supplanter are winners

[ahew]

what happens if the plane doesn't have jets? will the jets have enough power for Fx > Fy? Ambiguous.

blah... with all things perfect and Fx = Fy (according to this question) there will not be any movement.

I win... without need to touch Google.. hehehe.

somebody ask Dr. Karl please.

[scarecrow]

If the plane doesn't have jets, then it has propellers and the exact same thing occurs, it takes off.

The only way a plane would not take off under this situation is if the tyres popped from spinning at twice their normal rate, but even that is unlikely because once the plane hits those sorts of speeds it already has lift and there is very little pressure placed on the tyres.

No plane is propelled forward by its wheels like a car is.

Are you people dense or what? All you need to realise is that the plane's forward movement is independent of it's contact with the ground. The only effects a dynamically moving runway would have on takeoff are that the wheels would likely spin twice as fast as normal (in their usual direction) and because of the friction of the bearings within the wheels the plane might take another 10 - 50metres to take off, which is fine because runways are always longer then they need to be anyway.

[Bennoz]

lol you folks miss my first post?? Forget wheels, forget engines, forget the runway, forget exploding stewardesses. No air passing the wing - no flight.

Flying lesson 101 for kids

[afterburner]

Actually Ben I think they may be right - the plane would take off.

This is because the forward speed of the plane is not related to the wheels at all - essentially what the wheels are doing is completely independant of whether the plane flies or not. This is because the plane's method of forward movement is to act on the air around the plane, not on anything to do with the wheels...

[Bennoz]

But no lift is being created. There is no air passing the wing at speed...

[afterburner]

I guess it really comes down to whether you consider things like drag, friction, mass, etc in the equation.

What I mean by this is if you were in this situation (ie a plane on a massive treadmill), and instead of using the planes engines, you just started the treadmill in reverse, in the real world the plane would move backwards on the treadmill. But with a bit of application of the planes engines, it should stop moving backwards and stay still, while the wheels rotate on the treadmill - this power is essentially overcoming the friction of the wheel assemblies and the mass of the aircraft. However this should only require a finite amount of power, almost certainly less than the plane's full power. Therefore you should be able to move the rest of the thrust to move forward and take off. But of course then the treadmill starts moving faster and you need to apply more and more power just to stay in the same place, as the friction forces increase.

However in a world with no friction in the wheels, and not considering the mass of the plane as creating inertia, then I think the plane would take off. In this situation, the wheels could be moving backwards at 1,000,000kph, and the plane would sit still on the runway, and could still use the full force of the engines to move forward and lift off, because what is happening to the wheels is irrelevant to the plane's foward speed...

Actually now I think there might be a third option - the plane crashes in a spectacular wreck