Plane on a conveyor revisited Options

[jamie]

Its not that I dont understand what you are saying, you dont have to keep rewording the same thing.. Its just that it makes no differance if the power is coming from the wheels or not.

The plane can never break the cycle and 'overtake' the speed of the runway If it is being matched exactly.

If it was travelling at 2mph on the runway (pushed by its engines) and the conveyer belt was travelling at 2mph the plane would be still correct? At 10mph... the same, at 20...

At what speed would it magically over take the runway?

[james]

Imagine you are holding a matchbox car (the kind with the spinny wheels which just go around - not the kind with the wind up 'motor' in it) on a treadmill - by the logic you have outlined above you would not be able to push the car along the treadmill (and by push I mean holding on to it and moving your hand and therefore the model forward). But obviously you could do that - now substitute the plane for the car and the engine power for your arm and voila!

Even if the treadmill was going at a gazzillion miles an hour you could hold the car on it (the unpowered wheels would just rotate at the same speed as the treadmill). When you push it forward they would rotate at the speed of the treadmill plus the forward speed you are applying - they are for all intents and purposes irrelevant.

[Egg Designer]

Imagine you are holding a matchbox car (the kind with the spinny wheels which just go around - not the kind with the wind up 'motor' in it) on a treadmill - by the logic you have outlined above you would not be able to push the car along the treadmill (and by push I mean holding on to it and moving your hand and therefore the model forward). But obviously you could do that - now substitute the plane for the car and the engine power for your arm and voila!

Even if the treadmill was going at a gazzillion miles an hour you could hold the car on it (the unpowered wheels would just rotate at the same speed as the treadmill). When you push it forward they would rotate at the speed of the treadmill plus the forward speed you are applying - they are for all intents and purposes irrelevant.

But in this hypothetical situation, if no thrust was being applied and you turn the treadmill on, would the plane go backwards?

[jamie]

Exactly! Couldnt find a way of phrasing it.

The hand or rope pushing arnt fair comparisons as they bring in to effect an out side 'imovable' force. You might as well say what if you pick up the car and throw it.

And still, rewording it in another, yet exactly the same example is not going to change my mind!

[Emily]

I think this has got to the point where without seeing it you'll never believe, I totally get what your saying jamie and I thought just like you until somehow I saw the light. I know we are just going round in circles, I explain the same thing over than over then you explain the same thing again.

all I can say is imagine it didn't have wheels, what would happen then, in your mind?

[camsmith]

Jamie, I think you are forgetting one simple thing. Where does the thrust for a plane come from? The engines. What do the engines act on? The air (not the ground). Therefore there IS this outside force, as it is the air around the plane. The arm/rope is replaced by an engine (jet or propeller, doesn't matter) acting on the air around the plane. The air speed has nothing to do with the ground speed.

[jamie]

if the plane was floating i would agree.


There is still an opposing force from the wheels / runway. the wheels reduce this opposing force but do not remove it completely.

[camsmith]

If you want to take that line, then there is a similar force applied by the wheels of the rollerskates/blades on the treadmill with the rope. Nowhere near enough force to stop the object in question propelling itself through the air and in the case of the plane, taking off.

[jamie]

Ok, lets revisit the rope scenario

In a relevant analogy If you change one variable you would get the same result in both experiments correct?

scenario #1

Roller skates and rope

Cam is on his roller skates on a tread mill holding himself from going backwards. The treadmill stops, yet Cam is still going no where...

scenario #2

Plane on a runway

Whether you believe the plane would go forward with the treadmill on or not when you turn it off it should definitly go forward. Not stay still.

Totally differant results whichever way you look at it. How can this example be relevant?

By the way, I think we should all go down the pub and sort this out there. Im getting bored of typing!

This post has been edited by jamie: Mar 5 2008, 05:57 PM

[Jeffers]

Your wrong

Accept it and move on with your life.

Jeff

Ok, lets revisit the rope scenario

In a relevant analogy If you change one variable you would get the same result in both experiments correct?

scenario #1

Roller skates and rope

Cam is on his roller skates on a tread mill holding himself from going backwards. The treadmill stops, yet Cam is still going no where...

scenario #2

Plane on a runway

Whether you believe the plane would go forward with the treadmill on or not when you turn it off it should definitly go forward. Not stay still.

Totally differant results whichever way you look at it. How can this example be relevant?

By the way, I think we should all go down the pub and sort this out there. Im getting bored of typing!

[Emily]

I sorry I can't wait - Jamie you've missed out a key factor in your comment about the rope analogy not working, when your pulling on the rope and the tread mill stop you continue to move forward at the same rate as if the tread mill was on only your wheels would be spinning fewer times
this would be the same with the plane
and if your still not happy then forget that and answer my question from earlier, forget the wheels! what would happen without them?

This post has been edited by Emily: Mar 6 2008, 08:08 PM

[paul]

Did anyone see the Motor racing?

[jamie]

With or without the wheels the result would be the same. It would just take more force from the engines to maintain the same 'speed' as the opposing force / friction would be greater.

[Egg Designer]

The rope theory is not relevant in this argument as you are using a 'fixed' point to provide you with the ability to create motion.

If you had a conveyor, started it, and then started the plane engines to compensate this backward movement, surley the opposite would apply, in that if the plane started to progress down the runway, an increase in the speed of the conveyor belt would negate this?

This post has been edited by Egg Designer: Mar 12 2008, 12:13 PM

[Jeffers]

The plane will only take off if there is airflow over the wings.
If the plane is stationary then there is no airflow. It doesn't matter how fast its wheels are spinning.

Imagine standing behind the plane and pushing on it while the treadmill increased in speed. The wheels are doing Mach 4 but the plane is just sitting there as you are negating the tendency for the plane to move backwards. Massive speed in the wheels but the plane is not going to just start "flying". The only way the plane will lift off is if you start pushing on it enough to get it going forward at such a speed that lift was generated. This would then move the plane forward (as shown in the Mythbusters clip where the plane was moving through the air faster than the ground was moving underneath it) therefore providing lift.

That is the question is it not?
Will the plane take off? (While it is on a treadmill)

The answer is no if there is no forward movement providing airflow which in turn provides lift.

Now you could make it lift off the ground with no engines while the plane was stationary and chained to a post but you would need some seriously big fans ( a la Wind tunnel) to provide the airflow over the wings. It would then be, in effect, a giant kite.

Jeff