Physics Question

[juiio]

Ok, I am about ready to agree with this. The plane can not take off and stand still.

No one is claiming it can (other than Row1and, who is completely wrong).

It takes off because it is still moves forward.

 

[Armitage]

Originally posted by: Row1and

Originally posted by: Armitage

Originally posted by: edro

Originally posted by: Armitage

Originally posted by: Row1and
yes it can cause the speed of the plane is increasing. As soon as the plane gets to it's require Knots, it will take off. so yes it can take off, but it will be on the same spot. That would actually be cool to see i think,

No - the plane has to be moving relative to the earth/air to take off. And it will be.

You are missing the whole point of the question...

I don't think so - Row1and says it will take off, but will stay in the same spot. A convential plane can't do that. The whole wind over the wings everybody keeps on about.

It will take off, but not in the manner Row1and describes.

no, the plane will take off but it will look like it's taking off from a stand still position from someone looking at it far away. That's what I meant by take off but will be in the same spot.

Why would it look any different from "far away" then any other takeoff?

 

[letdown427]

congratulations to edro for being the first poster to resort to insults in this entire thread. "are you on crack?" is the bit i mean. meh. Anyway.

We all knkow that thrust doesn't equal lift. THrust does equal forward motion however, and that forward motion equals airflow over the wings which will generate lift.

Consider a firework? OK? please, consider a firework.

Imagine you've put some little wheels on it. or big wheels, it's up to you, but fact is, they're just inanimate wheels. if u spin them, they'll spin for a bit. that's it. ok? do we have that in our minds now?

So, let us pretend that this rocket has some minor aerodynamic foil that prevents it from taking off, and keeps it on the ground.

Now, if you lit that firework, and it was on the conveyer in this question, do you honestly think that it wouldn't move? Seriously? That is what you are argueing saying the plane won't take off.


Imagine someone shot a bullet at you, there is no rifling in the barrel, it just travels at the same orientation, it doesn't spin round. Pointlessly, they've stuck some little wheels on the bottom of it. For fun. Meh, let's say it's going through a vacuum, so there's no air resistance. According to your thinking, if this bullet (say it's travelling at a mere 500mph) with its little wheels on, was to have its wheels meet a treadmill going in the opposite direction at 500mph, that it would be held in place? That is ridiculous.


Go pick up a toy car. Now. DO IT.

Holding it in your right hand and hold your arms out to either side, bring your arms towards each other at the same speed, and run the wheels of that toy car over your left palm. Notice that effectively, the car is briefly on a treadmill going in the opposite direction when its wheels run over your palm. Does this stop it from moving? No. it doesn't. At all. it just makes the wheels spin faster. This is because the force moving the toy car forward, your right hand, is independent of your left hand. Like the thrust of a plane and the conveyer.

Honestly, i can't think of a simpler analogy.

 

[Chryso]

Ok, am I right in thinking that the conveyor and the wheels of the plane will be constantly increasing to an infinite speed as the plane begins to move forward until it is no longer on the conveyor or it leaves the ground?

 

[juiio]

Skateboard picture

Here's another one...

What happens when the fan is plugged in? Does the skateboard move?

(Pretend that the cord doesn't impede movement at all)

 

[giantpinkbunnyhead]

Originally posted by: Chryso
Ok, am I right in thinking that the conveyor and the wheels of the plane will be constantly increasing to an infinite speed as the plane begins to move forward until it is no longer on the conveyor or it leaves the ground?

Sort of. The wheels and will be moving twice as fast as normal. Say the belt moves back at 100 knots and the plane is moving forward relative to the earth at 100 knots. That's a 200 knot difference being noticed by the wheels. The belt and wheels won't accelerate to infinity because the acceleration stops once the plane is airborne. The wheels will be moving at twice the speed of the belt.

 

[letdown427]

no. the conveyer speed is relative to the plane. If it was relative to the wheels, it would lead to a paradox, whereby they instantly accelerated to an infinite speed. even in magic physics land that isn't the case.

the speed of the conveyer is relative to the plane. everyone must accept that as it is in the question. it is in one of the defined parameters of the question. no-one can realy deny that ok? simple.

The speed of the plane is relative to bob, the air, that guy next to the runway, the casual observer. The speed of the plane can't be relative to the conveyer.

Would ou like to know why it isn't relative to the conveyer?

Let's pretend it is. Plane goes at 1mph. conveyer now goes at 1mph also yes? in opposite direction. so now, relative to the conveyer, the pane is in fact going at 2mph. so now conveyer is at 2mph. but holy swiss cheese batman, plane is now relative to conveyer going at 4mph. this will lead to a paradox. so yes, plane speed is relative to some observer in the controltower let's say.

So, we've accepted that the speed of the conveyer is relative to the plane, as the question says so, and we've accepted that the speed of the plane is relative to the the observer, as if it was relative to the conveyer, we meet a paradox.

So, for your conveyor to be moving AT ALL, then the plane must be moving already.if it is moving, then it is clear that the conveyer can't stop it from accelerating as it had to accelerate from rest. The conveyer can't speed up to try to oppose the plane and stop the plan from speeding up, because the conveyer speed is relative to the plane, so if the conveyer has sped up, the plane must have too.

You cannot argue theplane can't take off and mention the convyer moving. Your only argument is as armitage constantly says, to prove that the non moving conveyer has a force that can oppose the thrust of the engines.
so give us one.

 

[giantpinkbunnyhead]

Originally posted by: juiio
Skateboard picture

Here's another one...

What happens when the fan is plugged in? Does the skateboard move?

(Pretend that the cord doesn't impede movement at all)

This is the wrong analogy. the fan is sitting directly on the skateboard, so there is a large amount of friction present. An airplane does not sit directly on the runway (or conveyor). It's on WHEELS. Try putting the fan on wheels and then put that assembly on the skateboard. Either the fan will push itself off the back of the skateboard, or if the fan is fixed in position (as the conveyor belt would be) then it would push the skateboard out from under it.

The wheels is what makes the difference here because they remove 99% of the friction that would otherwise keep the plane form moving.

 

[Armitage]

Originally posted by: edro

Originally posted by: DaiShan

Originally posted by: notfred
This thread's already about 3 million posts long, but I have to give my input. The plane WILL take off. The engines of an airplane move it relative to the air around it, not relative to the ground, like many people have said before.

Let's make an analogy. Pretend the plane is propelled forward by a giant winch. There's a rope attached to the front of the plane, and that rope is attached to a winch at the far end of the conveyor. Now, when the winch starts winding in and pulling the plane forward, do you think it matters how fast the conveyer under the plane moves? It doesn't the winch is going to keep pulling in rope at the same rate, and move the plane forward. The only thing that the conveyer will change is the rotation speed of the wheels, but the plane will keep moving forward at a constant rate because it's being pulled by a rope.
The engine on an airplane is like the rope, except the rope and the winch is air instead the engine pulls the plane through the air, independent of the movement of the ground.

You're right. I may have said differently earlier (dunno if I posted) but yeah, you're right.

You are on crack. You think what makes an airplane fly is the engine alone? You MUST have airflow across a wing to create lift... it is that simple.

Originally posted by: Chryso

Originally posted by: Armitage
One more time ...
All the naysayers - please tell me what force opposes the thrust of the engines to keep the plane stationary?

Thrust does not equal lift. You need airflow over the wings for lift.
I could put a jet engine on a car but that won't make it fly.

Exactly.

Originally posted by: Armitage

Originally posted by: Chryso

Originally posted by: Armitage
One more time ...
All the naysayers - please tell me what force opposes the thrust of the engines to keep the plane stationary?

Thrust does not equal lift. You need airflow over the wings for lift.

No kidding? Hang with me here!
So - if we have thrust...
And no force opposes that thrust...
The plane moves.
If the plane moves...
There is airflow over the wings.
If there is airflow over the wings..
There is LIFT!

I could put a jet engine on a car but that won't make it fly.

But it WOULD make if move forward, relative to the earth, on the conveyer! Which a conventional car could not do given the scenario in the OP.

If the plane DOES move forward in relation to the stationary Earth... it completely defeats the purpose of the question. The whole point is that the plane is not moving in relation to the Earth.

Please quote the first post in the discussion and show us exactly where it says this.

If you have a conveyor moving at 10mph and an engine propelling the plane @ 200mph, of course it will take off.
Read the question... it says that the conveyor MATCHES the thrust exactly, thus, no movement and no airflow over the wing.

I think you need to re-read the question - it never mentions anything about thrust. Just speed.

The conveyer CANNOT contribute any force in opposition to the engine thrust - with the exception of the wheel rolling resistance which we've already established doesn't differ significantly from that of a normal takeoff.

 

[juiio]

This is the wrong analogy. the fan is sitting directly on the skateboard, so there is a large amount of friction present. An airplane does not sit directly on the runway (or conveyor). It's on WHEELS. Try putting the fan on wheels and then put that assembly on the skateboard. Either the fan will push itself off the back of the skateboard, or if the fan is fixed in position (as the conveyor belt would be) then it would push the skateboard out from under it.

The wheels is what makes the difference here because they remove 99% of the friction that would otherwise keep the plane form moving.

I wasn't saying that it is the same. I already know that the plane takes off. I was just saying that it is something else that we could discuss.

 

[letdown427]

but that isn't very interesting as we all know that that wouldn't work.

 

[juiio]

Originally posted by: letdown427
but that isn't very interesting as we all know that that wouldn't work.

Bzzzt! Try again.

 

[dxkj]

I havnt read all of the crazy responses in this thread yet so this may have already been pointed out...



First look at how the conveyer belt acts on the plane.... Assume that the conveyer belt is moving 300 mph backwards..... I would not imagine that the plane would be moving in reverse at 300 mph.... assume that the wheels are almost perfectly frictionless.... the plane would remain mostly stationary and when thrust is applied it would move forward, and the wheels would spin at 600 mph.

Another way to look at it, the plane is sitting directly on the coveyer belt, the body right on it... the 300 mph belt moves it backward at 300mph, but as soon as friction is broken it would accelarate from -300mph to whatever speed the thrust would put it at.

 

[dxkj]

100 mph of the conveyer moving backwards would only translate into a net few mph because of the minimal amount of friction on the wheels

so if the conveyer is moving at 100mph plane puts enough thrust to move it at 100mph. in reality the plane will be moving 99 mph forward or something similar

 

[letdown427]

Originally posted by: juiio

Originally posted by: letdown427
but that isn't very interesting as we all know that that wouldn't work.

Bzzzt! Try again.

Either you're going to tell me that the white board falls over or blows away, or that that somehow works, and the skateboard moves.

The only way I can see that moving is the skateboard would move backwards(to the fan end) becaus the white board wouldn't capture all of the air?

Otherwise it would seem on first impressions that surely it can't be propelled forwards (towards the white sheet) as it would seem that the fan blowing air forwards would push the skateboard backwards, and this would be counteracted by the air pushing the white panel, thus pushing the skateboard forwards, and cancelling it all out.

I presume you have seen this in a lecture or something and it in fact works contrary to first impressions?

Meh, my firs tthought would be either it stays still in theoretical physics world, but RL it moves slightly backwards (fan end) as the white sheet doesn't feel the force of all the air.

Please enlighten me!


And yes, whilst you can try and thread jack (the 0.999...=1 jacking has been beaten) ultimately, people will read the topic and bluntly propse their it cant take off opinion without reading the thread, and it will eventually be steered back on course

 

[Chryso]

I saw Bugs Bunny do it to get away from Elmer Fudd.

 

[letdown427]

Originally posted by: Chryso
I saw Bugs Bunny do it to get away from Elmer Fudd.

O. ok. point taken

 

[So]

Originally posted by: letdown427

Originally posted by: juiio

Originally posted by: letdown427
but that isn't very interesting as we all know that that wouldn't work.

Bzzzt! Try again.

Either you're going to tell me that the white board falls over or blows away, or that that somehow works, and the skateboard moves.

The only way I can see that moving is the skateboard would move backwards(to the fan end) becaus the white board wouldn't capture all of the air?

Otherwise it would seem on first impressions that surely it can't be propelled forwards (towards the white sheet) as it would seem that the fan blowing air forwards would push the skateboard backwards, and this would be counteracted by the air pushing the white panel, thus pushing the skateboard forwards, and cancelling it all out.

I presume you have seen this in a lecture or something and it in fact works contrary to first impressions?

Meh, my firs tthought would be either it stays still in theoretical physics world, but RL it moves slightly backwards (fan end) as the white sheet doesn't feel the force of all the air.

Please enlighten me!


And yes, whilst you can try and thread jack (the 0.999...=1 jacking has been beaten) ultimately, people will read the topic and bluntly propse their it cant take off opinion without reading the thread, and it will eventually be steered back on course

No, what he's saying is that your metaphor doesn't apply to the physics question at hand. The engines aren't acting on the conveyor belt, but the air around the plane.

 

[JujuFish]

Originally posted by: edro
If the plane DOES move forward in relation to the stationary Earth... it completely defeats the purpose of the question. The whole point is that the plane is not moving in relation to the Earth.

Therein lies the brainteaser aspect. The statement is made to make you THINK that the plane is stationary. You will see, however, upon closer inspection, that it does not say that. The plane moves forward and takes off.

 

[juiio]

Originally posted by: letdown427

The only way I can see that moving is the skateboard would move backwards(to the fan end) becaus the white board wouldn't capture all of the air?

Bingo :thumbsup:

It isn't relevant to the plane question (the plane does take off), but I didn't see any reason to start another physics thread.

 

[mercanucaribe]

Originally posted by: SophalotJack
So, you have to be kidding right?

There is a massive difference between moving a vehicle forward on a reverse-moving plane compared to that same vehicle taking off.


A jet powered car would do the same thing.... but we know for sure that it wouldn't take off because it's not an airplane.

Don't let the "wings" fool you all... you need "lift." Which can only be attained if wind is hitting the wings. Seeing as though the only wind generated is behind the airplane (from the jet engines), there is no lift.


The thing that the people (who think this airplane might take off) are severely not understanding is that just because a plane moves forward (countering the reverse covneyer belt) doesn't mean that the airplane's wings are getting any lift. The air in front of the plane is still perfectly still (except the air immediately around the jet intakes, and that air doesn't get to the wings at the intake speeds).


Maybe if there was a hurricane force wind blowing toward the airplane.... then it might take off.


Man, how to people not get this?

I suppose parachutes can open up on the ground based on the way most people think.

The speed of the plane is independent of the wheels. They are free spinning. The conveyor belt can spin as fast as it wants, and the plane wheels will just spin faster.

 

[Rayden]

I repeat, can someone use physics to prove that the plane doesn't move? I have seen many people say it doesn't move, but no one uses physics in their reasoning.

 

[juiio]

Originally posted by: Rayden
I repeat, can someone use physics to prove that the plane doesn't move? I have seen many people say it doesn't move, but no one uses physics in their reasoning.

There's a reason no one can use physics to prove that it doesn't move

 

[ta8689]

People... the conveyor belt does not stop the plane from moving, it just makes the wheels spin faster. They can break off and skid, but the plane will move foreward. Think of it the same as if you were sitting on a computer chair with wheels. And somehow magically if you get going 5 mph you will take off. Well... you arent spinning the wheels. You are pulling a rope. The wheels will spin really fast, but you can always pull yourself with that ropeand get going. The air is the rope, and the engine is your arms. Make sense yet?

 

[kd2777]

fvck no it won't take off, there will be no lift.

Tire speed has nothing to do with a plane.

I can't believe "your group" was torn down the middle on this.

kd