*cough*

For the love of God! Cam - read the first post about the conveyor belt counteracting the forward motion provided by the thrust of the jets, therefore making the groundspeed of the plane effectively zero, we are not debating friction or the jets ability to overcome the effects of the conveyor belt, the jets on a plane only provide thrust, not lift - as we all know - but you need speed across the ground to achieve the lift. If in the case we are looking at we take it that the surrounding air is stationary (hypothetically) A plane needs to have air moving at speed underneath its wings to take of, the above example has a planes forward motion, in theory, at zero, therfor zero lift created.

Basic physics - just the same as beating with a stick! :)

Ok this is a conversation I had from a colleague that has an IQ as high as a very high thing and some say he is a tame alien with a brain the size of a planet.


[09:01] Mike (Skymarket): "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?"

[09:06] hairytart: The plane will take off when its air speed reaches a certain amount. What isn't made clear here is whether the plane is moving relative to the conveyer (therefore not moving relative to the planet and the atmosphere) or whether the plane is moving but because of the actions of the conveyer its wheels are spinning twice as fast as they should be.

[09:12] Mike (Skymarket): i would have said yes it will take off as the wheels are just free wheeling (little or no friction) and that the wheels are not driven. The thrust is provided by the jet engines so as long as they have more thrust than friction the plane will always move forward. The wheels would only free wheel twice as fast as if they were on the fixed flooring.

[09:13] hairytart: yes I agree
The unclear bit is whether that was what the question actually meant
Itís a badly worded question really

If the plane was wheel driven (clearly a daft idea) then the question might have had more validity, though the plane would tend to slow down the moment it left the tarmac!

And before you ask yes he is hairy and his surname is Tart!

Rory - I've run out of ways to explain how you are wrong!

Mike - your mate is quite correct, the wording is not 100% perfect, but if it was then wehre would all the fun of people getting it wrong come from

can i ask the everyone read my post about the wheels and the rope and explain to me how that wouldn't work then when you've done that i will believe you.

imagine you have wheels, and you have a big rope in front of you (the rope is the air) if you pull yourself along the rope it doesn't matter how fast the conveyer is going you will move along the rope, the wheels will just spin faster to keep up with you even if the conveyer spins faster too. you plane is taking air from the front and putting it behind itself in the same way you pull in the rope and push it away behind you

This post has been edited by Emily: Feb 28 2006, 09:41 AM

Yeah id guess that close to how it works Em. As long as you can pull harder on the rope than the effect of any friction on the wheels yeah you would move forward. If the rope was the air then this would be flowing over the wings to cause lift.

If this question was following all the laws of physics (which it isnt) then as the plane gave a little thrust (equal to the friction + drag) it would not be moving forward but once the thrust becomes greater than the friction + drag the plane will move forward. The conveyor would end up only doing half the speed of the wheels.
As the plane would be moving forward in this scenario there would be air moving over the wing so the plane would take off.

But in this fictional scenario they seem to have fictional physics as well. "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)".

This suggests that the friction and drag increase at the same rate as the planes thrust so that the thrust never overcomes the friction thus never moves forward.
In this scenario the plane will appear to be not moving thus not having any air over the wings therefore not taking off.

This post has been edited by womble: Feb 28 2006, 10:39 AM

I don't understand that post Mike - on a freewheeling system why would increasing the speed of the conveyor increase the friction or drag (apart from so minutely as to make no difference to the model)?

The rope pulling example Em gave above is probably the best analogy I can think of too:
The rope is the motive force (like the engines pushing air out the back of the plane) and as the friction of the wheel bearings is negligible once inertia has been overcome.
The force exerted by the engines (or pulling the rope) will be greater than any friction in the wheels - thats why they have wheels.
The fact that the conveyor speeds up is also irrelevant (so many people fixating on the irrelevant parts of the question) - try this example:

Using the treadmill, say it starts running at 20mph and you pull yourself forward at 2 mph, depending on how often the feedback system updates the speed controller in the conveyor, the treadmill speed will go up, but that makes no difference to you pulling yourself along the rope.
Now try it with the treadmill at 100mph, or 500mph - or whatever. Unless the wheels seize up they will always spin around at whatever the speed of the treadmill is plus your forward velocity caused by pulling on the rope, but none of that matters to what happens on the rope.

My answer was in 2 parts, one being with real physics applied to what would happen and the other was applying real physics to make the fictional question actually happen.

The increase in friction is what would have to happen for the fictional scenario of the belt being able to keep up with the wheels. (trying to apply some basic physics to a fictional scenario to make it happen)
If thrust is greater than friction then you have forward movement. Therefore for the belt to keep the plane at standstill as the plane gave more thrust something else would have to increase as well,(velocity wouldnt do it on its own as the wheel would freewheel (no friction) So we have to apply friction between the wheels and the belt and this friction would have to increase at the same rate as the thrust increased. with that we get friction equals thrust and no movement.


You could look at it another way in that the engines are so small that they can only produce enough thrust to match friction thus it would appear not to move. If this was the case then the plane wouldnt take off even if it was on a normal runway.

This post has been edited by womble: Feb 28 2006, 10:39 AM

OK, I haven't replied to this for a little while.

Rory, Rory, Rory... Where do I begin.

The words are NOT that the conveyor belt moves fast enough to counteract the jets... it simply moves in the opposite direction at the same speed as the plane. YOU are the one who is assuming (quite wrongly) that this is enough to counteract the forward motion. It is not. It is simply making the wheels spin twice as fast.

The plane moves through the air, the ground is simply there to hold it up until the wings can take over.

If you like, I could show you where to put the stick

Well yeah I understand what you are saying but as the question is about a plane which by definition has enough thrust potential (fnarf) to reach takeoff velocity, so as you said in the last paragraph -
so I'm not sure what you are driving at? Unless you were just commenting on a hypothetical situation outside of the scope of the original question in which case yes you are absolutely right.

Rory - regarding - by the logic of what you have said the conveyor would need to be applying thousands of pounds of force in the oposite direction to the direction of the engines - but as friction in the frewheeling wheels is almost negligable, increased speed on the conveyor does not translate to force against the plane.

Yay you got it, its hypotheticaly what would need to change to make the belt be able to hold the plane back and still be in the bounds of real physics. (you either increase the friction to above the thrust or decrease the thrust to below the friction).
Both of which would make an abnormal plane that wouldnt fly even under normal runway conditions.

This post has been edited by womble: Feb 28 2006, 10:57 AM

Anyone see the footie last night?????

I bet all you saw was wedding books!

big stick immenent

OK, I understand that forward thrust in the real world, yes the plane will move forward and take off, but I am taking the question that they suugest that the planes thrust is being countered by the conveyor belt, in the real world, the free hubs of the planes gear will spin twice as fast and the plane will go uppity wards! I

Just taking the question as a badly worded thing to create debate I guess!

ok get the sticks out, its the hypotheticals vs. real world conveyeor belts!