Physics Question

[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.

 

[Armitage]

Originally posted by: Chryso

Originally posted by: Rayden

Originally posted by: Chryso

Originally posted by: Rayden
THE PLANE MOVES! The conveyor belt does note exert any force on the plane! All it does is spin the wheels. NOWHERE does it say that the conveyor belt exerts any force on the plane SUFFICIENT to keep it from moving. How are you guys inferring that?

If you wish to argue that the plane does not move, please use some physics and prove that the force of the plane's engine equals the force from the conveyor belt.

Why do you think the plane moves? This is from the original post:
"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)"

Where does it say that the plane doesn't move? It says the conveyor moves backwards with the same speed that the plane moves forward. How does the conveyor moving affect the plane?

That is pretty much the definition of not moving.
Let's say that the plane is going 100mph and the conveyor is going 100mph in the other direction. 100 - 100 = 0.

You have some coordinate frame confusion here.
If the planes speed is measured relative to the conveyor belt, what you said above is true.

But the planes engines move the plane relative to the earth.

In an earth-fixed coordinate frame:
Speed of Plane = Vp = 100
Speed of Conveyer = Vc = -100

To convert this to the conveyor coordinate frame you have to subtract the speed of the conveyor relative to the earth coordinate frame:

Vp_ccf = Vp - Vc = 100 - -100 = 200

 

[dawks]

Originally posted by: Chryso

Originally posted by: Rayden

Originally posted by: Chryso

Originally posted by: Rayden
THE PLANE MOVES! The conveyor belt does note exert any force on the plane! All it does is spin the wheels. NOWHERE does it say that the conveyor belt exerts any force on the plane SUFFICIENT to keep it from moving. How are you guys inferring that?

If you wish to argue that the plane does not move, please use some physics and prove that the force of the plane's engine equals the force from the conveyor belt.

Why do you think the plane moves? This is from the original post:
"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)"

Where does it say that the plane doesn't move? It says the conveyor moves backwards with the same speed that the plane moves forward. How does the conveyor moving affect the plane?

That is pretty much the definition of not moving.
Let's say that the plane is going 100mph and the conveyor is going 100mph in the other direction. 100 - 100 = 0.

The wheels are going 100mph in the other direction, the plane is free to move forward how ever much it likes. Just apply the throttle to 'pull' it forward.

 

[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.

 

[Chryso]

Originally posted by: Armitage

Originally posted by: Chryso

Originally posted by: Rayden

Originally posted by: Chryso

Originally posted by: Rayden
THE PLANE MOVES! The conveyor belt does note exert any force on the plane! All it does is spin the wheels. NOWHERE does it say that the conveyor belt exerts any force on the plane SUFFICIENT to keep it from moving. How are you guys inferring that?

If you wish to argue that the plane does not move, please use some physics and prove that the force of the plane's engine equals the force from the conveyor belt.

Why do you think the plane moves? This is from the original post:
"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)"

Where does it say that the plane doesn't move? It says the conveyor moves backwards with the same speed that the plane moves forward. How does the conveyor moving affect the plane?

That is pretty much the definition of not moving.
Let's say that the plane is going 100mph and the conveyor is going 100mph in the other direction. 100 - 100 = 0.

You have some coordinate frame confusion here.
If the planes speed is measured relative to the conveyor belt, what you said above is true.

But the planes engines move the plane relative to the earth.

In an earth-fixed coordinate frame:
Speed of Plane = Vp = 100
Speed of Conveyer = Vc = -100

To convert this to the conveyor coordinate frame you have to subtract the speed of the conveyor relative to the earth coordinate frame:

Vp_ccf = Vp - Vc = 100 - -100 = 200

I read the original post as saying that the plane was not moving at all relative to the earth.

 

[juiio]

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.

The plane is moving.

Say the wheels are locked and the plane is on a sheet of ice. Will it ever take off?

 

[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,

 

[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.

 

[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.
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.

 

[Row1and]

crap, i posted and didn't know it had already been answered. that's ma bad.

 

[juiio]

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,

That isn't right either. The plane doesn't stay in the same spot.

 

[Armitage]

Originally posted by: Chryso

Originally posted by: Armitage

Originally posted by: Chryso

Originally posted by: Rayden

Originally posted by: Chryso

Originally posted by: Rayden
THE PLANE MOVES! The conveyor belt does note exert any force on the plane! All it does is spin the wheels. NOWHERE does it say that the conveyor belt exerts any force on the plane SUFFICIENT to keep it from moving. How are you guys inferring that?

If you wish to argue that the plane does not move, please use some physics and prove that the force of the plane's engine equals the force from the conveyor belt.

Why do you think the plane moves? This is from the original post:
"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)"

Where does it say that the plane doesn't move? It says the conveyor moves backwards with the same speed that the plane moves forward. How does the conveyor moving affect the plane?

That is pretty much the definition of not moving.
Let's say that the plane is going 100mph and the conveyor is going 100mph in the other direction. 100 - 100 = 0.

You have some coordinate frame confusion here.
If the planes speed is measured relative to the conveyor belt, what you said above is true.

But the planes engines move the plane relative to the earth.

In an earth-fixed coordinate frame:
Speed of Plane = Vp = 100
Speed of Conveyer = Vc = -100

To convert this to the conveyor coordinate frame you have to subtract the speed of the conveyor relative to the earth coordinate frame:

Vp_ccf = Vp - Vc = 100 - -100 = 200

I read the original post as saying that the plane was not moving at all relative to the earth.

Again - what force opposes the engines such that it would remain stationary relative to the earth. The OP only mentions an airplane and a conveyer.

 

[Chryso]

Originally posted by: juiio

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.

The plane is moving.

Say the wheels are locked and the plane is on a sheet of ice. Will it ever take off?

The locked on ice is different because the plane can slide and cause airflow over the wings.

Let's say that I am standing next to the conveyor on which the plane is rolling.
In relation to my position, how is the plane moving?

 

[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.

 

[Gamer X]

Simple No

 

[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...

 

[juiio]

The locked on ice is different because the plane can slide and cause airflow over the wings.

Let's say that I am standing next to the conveyor on which the plane is rolling.
In relation to my position, how is the plane moving?

The plane is moving away from you, in the direction it is facing.

The ice example is no different. The wheels on a plane don't drive a plane. They spin freely without exuding any force on the plane. Heck the wheels could be spinning backwards, and the plane could sitll move forwards. The conveyer belt exudes force on the wheels, but the wheels don't exude any force on the plane.

 

[Armitage]

Originally posted by: Chryso

Originally posted by: juiio

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.

The plane is moving.

Say the wheels are locked and the plane is on a sheet of ice. Will it ever take off?

The locked on ice is different because the plane can slide and cause airflow over the wings.

Let's say that I am standing next to the conveyor on which the plane is rolling.
In relation to my position, how is the plane moving?

If the pilot hits the throttle, the plane will move relative to you. What force would oppose the thrust of the engines to keep it stationary?? Answer that, and you'll understand the problem.

 

[Chryso]

I think I can now see their side of the argument.
Hmmmm....

 

[juiio]

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...

The original question needs to be clearer. Instead of saying

"This conveyer has a control system that tracks the plane speed ",

it should say:

"This conveyer has a control system that tracks the plane's wheels angular velocity"

The plane's velocity and the plane's wheel's angular velocity are not the same. The plane will still be moving forward relative to an observer (the atmosphere) even if the conveyer belt is moving backwards at a velocity equal to the wheel's angular velocity. There is no force exuded on the plane by the plane's wheels.

But really, it doesn't matter how fast the conveyer belt is moving, because the wheels exude no force on the plane.

 

[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.

 

[Chryso]

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.

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

 

[Row1and]

this is getting pretty heated really quick. so what's the answer? will it take off or not?

 

[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.

 

[So]

Originally posted by: Row1and
this is getting pretty heated really quick. so what's the answer? will it take off or not?

Yes -- under ideal circumstances (i.e. near frictionless wheels). Anyone who says no has not thought it out, or doesn't have a solid grasp of physics.