You have a plane and a conveyor belt.

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: KK
Think about it, the conveyor belt will never keep up. This question has you believe that it will but no way in hell it could.

It doesn't matter if the conveyor belt can "keep up" or not, the plane takes off. Even if you set it up to move at twice the speed of the airplane in the opposite direction, the plane takes off.

Another example. The plane starts in the air, on a landing approach with the gear down. The conveyor is moving backwards at, oh, 300 MPH. The plane touches down, holding a constant throttle position, rolls to the end of the runway, and lifts off again. As it circles around to make the approach again, we switch the conveyor so it moves FORWARDS at 300mph. Once again our plane touches down, rolls to the end of the runway under constant throttle, and takes off. As long as the pilot didn't do anything silly like touch the brakes, the two approaches would appear identical to an observer on the ground. Maybe a few feet plus or minus, because of rolling resistance, but certainly nothing drastic.

If THAT doesn't demonstrate that groundspeed isn't airspeed, nothing will. Groundspeed (at least in the direction the plane's pointing) only affects airspeed when the brakes are applied!

It's funny, because in your attempt to give an example that proves your point, you actually prove mine !

In your example the conveyor belt has no effect on the speed of the moving plane. That is because the conveyor belt cannot transfer any force to the plane. The plane is moving because it has momentum, the conveyor belt prevents any force in the opposite direction from affecting this momentum.

In the same way, the plane at rest cannot move, because the special condition that it is resting on a device that will prevent any force from having an effect on it's present state at rest.

Again I will refer those that don't understand to my example(as I think it is the easiest to picture).

Take a toy car, wheels are not driven by any motor. Put it on a angled treadmill so that it would roll forward from rest. now turn on the treadmill to go in the opposite direction the car wants to roll in. Make it go as fast as you want but that toy car IS going to roll down the treadmill.

Now, some may say "HEY THAT'S UNFAIR! You're treadmill is declined." You must think of this in terms of forces. The engines apply a force to the plane. In much the same way, gravity applies a very similar force to the car on the treadmill. Both forces are not affected by treadmill speed. Therefore the substitution of gravitational force for the engines thrust is perfectly valid.

Hope that helps you guys picture it.

You keep trying to introduce external forces that don't exist in the original scenario. An inclined plane is an external force of gravity.

As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move releative to anythnig but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Tell me how the force of gravity acts different on the place than its own engines do

I didn't say it acts differently, i said it's an external force, whereas the engines are part of the system that includes the plane and the wheels, and the conveyor belt.

 

[PurdueRy]

Originally posted by: Tom

I didn't say it acts differently, i said it's an external force, whereas the engines are part of the system that includes the plane and the wheels, and the conveyor belt.

So if they act on the plane in the same way, as you acknowledge, what is wrong with the substitution of one for the other?

Both cause an unopposed force in the direction of travel, both are not affected by treadmill speed.

Refer to his diagram he made. Try to find the connection between the treadmill and the thrust of the engines. There is no connection, they are not related. The treadmill can only spin the wheels, it cannot move the plane. The engines can however, due to an unopposed force.

 

[jagec]

Originally posted by: blahblah99
1) If the earth was all land and everyone ran (in synch) to the east, would the earth spin faster and cause the days to be less than 24 hours?

2) Which will hit the ground first? A bullet fired from 2 feet high, or a bullet dropped from 2 feet high?

3) If you can travel faster than the speed of light, can you go around the world and come back to the same spot to see yourself start on the journey around the world?

1)No, the earth would spin SLOWER if they ran to the east. It would spin faster if they ran to the west But only as long as they kept running!

 

[Tom]

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

 

[JujuFish]

Originally posted by: Tom
If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

This is where you are mistaken. Once you realize why, you will then see that the plane takes off.

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

 

[jagec]

Originally posted by: Tom
And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

Ah, finally we're getting places!

The conveyor can affect the ROTATION of the wheels...but it cannot affect the MOVEMENT of the wheels. Which is, basically, the reason the wheel was invented...to keep friction with the ground from affecting the MOVEMENT of the wheel, and the load attached to it.

 

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

I didn't say it acts differently, i said it's an external force, whereas the engines are part of the system that includes the plane and the wheels, and the conveyor belt.

So if they act on the plane in the same way, as you acknowledge, what is wrong with the substitution of one for the other?

Both cause an unopposed force in the direction of travel, both are not affected by treadmill speed.

Refer to his diagram he made. Try to find the connection between the treadmill and the thrust of the engines. There is no connection, they are not related. The treadmill can only spin the wheels, it cannot move the plane. The engines can however, due to an unopposed force.

Because gravity, or a rope tied to a wall, are not part of frame of reference of the plane and the conveyor belt.

As I already said, the diagram is faulty, because it assumes the ground and all of the surroundings that the plane exist in are one constant fram of reference, but they are not, because in this scenario the ground moves relative to the rest of the enviroment.

Yes the engines exert a forward movement, and the plane moves forward relative to the conveyor, but simultaneously the conveyor moves in the opposite direction, at exactly the same speed. The net effect of these two combined is that the plane does not move relative to any other part of the enviroment, which includes the air.

 

[Tom]

Originally posted by: JujuFish

Originally posted by: Tom
If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

This is where you are mistaken. Once you realize why, you will then see that the plane takes off.

Except I'm not mistaken. Just saying I'm mistaken isn't much of an argument, btw.

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

I didn't say it acts differently, i said it's an external force, whereas the engines are part of the system that includes the plane and the wheels, and the conveyor belt.

So if they act on the plane in the same way, as you acknowledge, what is wrong with the substitution of one for the other?

Both cause an unopposed force in the direction of travel, both are not affected by treadmill speed.

Refer to his diagram he made. Try to find the connection between the treadmill and the thrust of the engines. There is no connection, they are not related. The treadmill can only spin the wheels, it cannot move the plane. The engines can however, due to an unopposed force.

Because gravity, or a rope tied to a wall, are not part of frame of reference of the plane and the conveyor belt.

As I already said, the diagram is faulty, because it assumes the ground and all of the surroundings that the plane exist in are one constant fram of reference, but they are not, because in this scenario the ground moves relative to the rest of the enviroment.

Yes the engines exert a forward movement, and the plane moves forward relative to the conveyor, but simultaneously the conveyor moves in the opposite direction, at exactly the same speed. The net effect of these two combined is that the plane does not move relative to any other part of the enviroment, which includes the air.

Everything is PERFECT up until there. You are absolutely correct, but how does the conveyor slow down the plane that is "moving forward relative to the conveyer"? What force opposes the engines force to deny the plane acceleration?

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom
If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

This is where you are mistaken. Once you realize why, you will then see that the plane takes off.

Except I'm not mistaken. Just saying I'm mistaken isn't much of an argument, btw.

It DOES matter that the wheels aren't driven. If the wheels were...the conveyor would be able to move the plane when it is at rest, it can't however. So a plane and a car(not a toy) act very different in this scenario.

 

[JujuFish]

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom
If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

This is where you are mistaken. Once you realize why, you will then see that the plane takes off.

Except I'm not mistaken. Just saying I'm mistaken isn't much of an argument, btw.

*sigh*
I was trying to let you figure it out yourself. Stop and think.
A car's engine turns its wheels which push off the ground, giving it forward movement.
A plane gets forward movement through the thrust of its engines with no dependency on the wheels at all. If planes needed to push off the ground with their wheels in the same way that cars did, they'd never stay in the air. They'd also never be able to take off on water.

 

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

Your toy car example does not illustrate anything related to the original question, it's a completely different situation. All you toy car example says is gravity makes things fall down, nobody disputes that.

However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

 

[KK]

Originally posted by: Tom

Originally posted by: KK

Originally posted by: Tom

Originally posted by: KK

Originally posted by: Tom

Originally posted by: KK

Originally posted by: Tom
If the conveyor belt always moves at the same speed that the wheels rotate, in the opposite direction, then the plane never moves relative to anything except the conveyor belt, so it doesn't move through the air.

Adding thrust will only increase the speed of the wheels and the conveyor belt, which will always cancel each other out.

It isn't the same as taking off on water or ice, in those cases the airplane does move relative to it's surroundings and the ground.

It also isn't the same as the rope attached to a wall, in this scenario there is no wall, and no rope, all there is increased thrust, which is exaclty the same as a man running on a treadmill. The man can run faster, but he won't move forward if the treadmill matches his speed in the opposite direction.

Think about it, the conveyor belt will never keep up. This question has you believe that it will but no way in hell it could.

What do you mean the conveyor belt can't keep up ? One of the premises of the original question is that the conveyor belt DOES keep up.

Given the wording of the question, there is no way for the wheels to move relative to anything but the conveyor belt. All forward movement of the wheels is completely balanced by backwards movement of the belt. It doesn't matter where the force that is making the wheels turn is coming from, increasing the force just makes the wheels and the belt turn faster.

And if the wheels can't move, neither can the plane.

But thats the thing, its impossible for the conveyor belt to keep up. The treadmill scenerio is along the same lines, if you are on a treadmill with roller blades on, and you have that rope attached to the wall in front, if you pull on the rope you will move forward. The treadmill will not be able to counteract the force you are pulling. Does that make any sense the way I'm trying to explain it?

But there is no rope or wall in the airplane scenario. Those are external forces not present. In the airplane scenario, there is no force that can act on the airplane that does not also act on the wheels and conveyor belt, simultaneously. The engines acting on the air, are going to apply a forward force to the plane, and necessarily the wheels, but all of that work will only make the conveyor go faster in the opposite direction.

And besides, the question isn't whether or not the conveyor belt CAN keep up, it is a given fact as part of the problem that it WILL keep up. Whether it makes sense in the real world or even in physics, is irrelevant.

The engine is the external force, whereas the rope & wall would be the external force in the treadmill scenerio. When you say that the engines are going to apply a forward force to the plane, it doesn't matter what the wheels are going to do as the engine pushes just the body of the plane. The wheels just spin freely in relationship to how fast the plane in going thru the air to whatever is happening on the ground.

The engine is not external, it is part of the plane. Your description of the wheels sounds crazy, how are the engines going to push the "body" without moving the wheels ?? And we already know as a condition that the wheels cannot move, any movement forward by the wheels is countered by the same movement in the opposite direction by the belt.

Forget this whole plane deal and lets go back to the treadmill situation. Tell me what happens if you have the treadmill programmed to counter act any forward motion you may make by adjusting the speed of the treadmill. What happens when you pull on the rope?

Certainly you can see that it's impossible to have the conveyor belt counteract your forward motion, therefore this question is just plain stupid. It's looking for a logical answer based on an impossible scenerio.

 

[jagec]

Originally posted by: Tom
As I already said, the diagram is faulty, because it assumes the ground and all of the surroundings that the plane exist in are one constant fram of reference, but they are not, because in this scenario the ground moves relative to the rest of the enviroment.

No, it doesn't. In my diagram, the "rest of the surroundings" are at a velocity of zero, the conveyor (which the plane is sitting on) is moving backwards (relative to the ground) at Vc, and the plane is moving forwards (relative to the ground) at Va. So the plane is moving forwards relative to the conveyor at Vc+Va.

 

[jagec]

Originally posted by: Tom
However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

Originally posted by: NanoStuff
The plane increases it's thrust and the wheels begin to rotate. The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward.

In other words, Va = Vc, where BOTH velocities are relative to the stationary ground. But Vc doesn't affect Va at all, it just matches it.

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

Your toy car example does not illustrate anything related to the original question, it's a completely different situation. All you toy car example says is gravity makes things fall down, nobody disputes that.

However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

You shoot down the example, yet you offer no proof of how it does not fit this scenario. You also don't answer how the conveyor causes a force to oppose the thrust of the engine.

We will be going in circles until you can come to the conclusion on these.

Gravity causes things to fall down, engines cause things to go forward, the result is the same in this case. Both cause an unopposed force forward resulting in the acceleration of the plane. You say it is completely unrelated, but if you would think about the FBD of this situation you would realize both illustrate the same situation.

 

[Tom]

Originally posted by: JujuFish

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom
If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

This is where you are mistaken. Once you realize why, you will then see that the plane takes off.

Except I'm not mistaken. Just saying I'm mistaken isn't much of an argument, btw.

*sigh*
I was trying to let you figure it out yourself. Stop and think.
A car's engine turns its wheels which push off the ground, giving it forward movement.
A plane gets forward movement through the thrust of its engines with no dependency on the wheels at all. If planes needed to push off the ground with their wheels in the same way that cars did, they'd never stay in the air. They'd also never be able to take off on water.

I did not say, and the original question does not require the plane to "push off" the ground using the wheels. The question isn't about what makes a plane fly, it is about understanding relative motion.

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

 

[PurdueRy]

Originally posted by: Tom

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

I fully grasp what you are struggling with and that's why we are trying to help you understand.

Explain to me this one situation:

The place in on the treadmill. The treadmill speeds up to 1 billion RPM(just for kicks). Neglecting friction, what happens? Nothing, the plane stays still.

Now with the treadmill rotating like crazy, turn on the engines. What happens? Well the treadmill is already going much faster than the plane, it has no affect on the planes speed relative to ground. The engines cause an unopposed force relative to ground and the plane will begin to creep forward.

The treadmill can spin as fast as it wants, just like in the toy car example, it cannot change the planes speed relative to ground...which is what matters.

 

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

Your toy car example does not illustrate anything related to the original question, it's a completely different situation. All you toy car example says is gravity makes things fall down, nobody disputes that.

However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

You shoot down the example, yet you offer no proof of how it does not fit this scenario. You also don't answer how the conveyor causes a force to oppose the thrust of the engine.

We will be going in circles until you can come to the conclusion on these.

Gravity causes things to fall down, engines cause things to go forward, the result is the same in this case. Both cause an unopposed force forward resulting in the acceleration of the plane. You say it is completely unrelated, but if you would think about the FBD of this situation you would realize both illustrate the same situation.

As I said before, if the original question's premise is maintained, about the conveyor movement always matching the wheel movement, in the opposite direction, then in either the original case, or your gravity case, the car will only move relative to the conveyor.

So I'm only shooting down your example because it doesn't change anything, not because it refutes my point.

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

Your toy car example does not illustrate anything related to the original question, it's a completely different situation. All you toy car example says is gravity makes things fall down, nobody disputes that.

However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

You shoot down the example, yet you offer no proof of how it does not fit this scenario. You also don't answer how the conveyor causes a force to oppose the thrust of the engine.

We will be going in circles until you can come to the conclusion on these.

Gravity causes things to fall down, engines cause things to go forward, the result is the same in this case. Both cause an unopposed force forward resulting in the acceleration of the plane. You say it is completely unrelated, but if you would think about the FBD of this situation you would realize both illustrate the same situation.

As I said before, if the original question's premise is maintained, about the conveyor movement always matching the wheel movement, in the opposite direction, then in either the original case, or your gravity case, the car will only move relative to the conveyor.

So I'm only shooting down your example because it doesn't change anything, not because it refutes my point.

So the car won't move relative to ground in the toy car case!?!

 

[jagec]

Originally posted by: Tom
I did not say, and the original question does not require the plane to "push off" the ground using the wheels. The question isn't about what makes a plane fly, it is about understanding relative motion.

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

Reference points are arbitrary. I can pick my reference point as a yellow dwarf 500,000 light-years away, which is currently moving away from the plane at a significant fraction of C, and the plane will still accelerate forwards and take off.

The only "speed" that matters in this problem is the speed of the plane relative to the air. The air is stationary. The plane exerts a force Fa (on the AIR, mind you) to propell itself forwards to Va (relative to the AIR). The conveyor senses this speed change, and exerts a force on the wheel of the plane Fc, while moving at Vc (relative to the AIR). The wheel starts spinning, but since the SPIN of the wheel doesn't affect the FORWARD MOTION of the wheel relative to the AIR, the plane continues to move down the conveyor to an on-time departure.

 

[PurdueRy]

Originally posted by: jagec

Originally posted by: Tom
I did not say, and the original question does not require the plane to "push off" the ground using the wheels. The question isn't about what makes a plane fly, it is about understanding relative motion.

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

Reference points are arbitrary. I can pick my reference point as a yellow dwarf 500,000 light-years away, which is currently moving away from the plane at a significant fraction of C, and the plane will still accelerate forwards and take off.

The only "speed" that matters in this problem is the speed of the plane relative to the air. The air is stationary. The plane exerts a force Fa (on the AIR, mind you) to propell itself forwards to Va (relative to the AIR). The conveyor senses this speed change, and exerts a force on the wheel of the plane Fc, while moving at Vc (relative to the AIR). The wheel starts spinning, but since the SPIN of the wheel doesn't affect the FORWARD MOTION of the wheel relative to the AIR, the plane continues to move down the conveyor to an on-time departure.

Precisely :thumbsup:

 

[JujuFish]

Originally posted by: Tom
I did not say, and the original question does not require the plane to "push off" the ground using the wheels. The question isn't about what makes a plane fly, it is about understanding relative motion.

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

The water does have bearing, because it's a simple way to show that a plane's forward movement does not require the use of wheels.

Let's start with what we can agree on: the plane has to move forward relative to the ground in order to achieve flight. Okay? Good.

Your argument is that the wheels are not moving relative to the ground and therefore the plane, because it is attached to the wheels, must therefore also not be moving relative to the ground. If the former were true, then you'd be correct. However, that is not the case. The situation formed in the OP does not necessitate that the the wheels be stationary relative to the ground. If anything, it requires the wheels to move forward.

Originally posted by: NanoStuff
The plane increases it's thrust and the wheels begin to rotate. The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward.

The conveyor belt does not prevent the wheels from moving forward. They just move in the opposite direction with the same speed. This is where I believe you've misunderstood the problem. In your interpretation, the wheels remain fixed while the conveyor belt moves, all relative to the ground. In actuality, the conveyor belt moves backward as much as the wheels move forward, thereby matching its speed. Thus, the wheels ARE moving forward relative to the ground, and consequently the plane takes off.

If you're still not convinced, look at the converse of the situation you've created in your mind, and you might see the flaw: The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels. Obviously, this is not the case, but this is the same way you're interpretting the original scenario.

Edit: Editted to kill the nested quotes.

 

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

I fully grasp what you are struggling with and that's why we are trying to help you understand.

Explain to me this one situation:

The place in on the treadmill. The treadmill speeds up to 1 billion RPM(just for kicks). Neglecting friction, what happens? Nothing, the plane stays still.

Now with the treadmill rotating like crazy, turn on the engines. What happens? Well the treadmill is already going much faster than the plane, it has no affect on the planes speed relative to ground. The engines cause an unopposed force relative to ground and the plane will begin to creep forward.

The treadmill can spin as fast as it wants, just like in the toy car example, it cannot change the planes speed relative to ground...which is what matters.

It doesn't matter how fast the treadmill is going, what matters is it will ALWAYS go exactly the right speed to keep the wheels from moving relative to anything but the conveyor, that is the FACT we are given.


"The engines cause an unopposed force relative to ground and the plane will begin to creep forward. "

EXACTLY WHAT I HAVE BENN SAYING ALL ALONG. THE PROBLEM IS THE CONVEYOR BELT IS THE GROUND.

The plane doesn't have to creep forward, it can accelerate like a scalded ape. But all of the motion is irrelevant to it's movement through the air, because in this case the ground is not tied to the air, it is in fact tied to the movement of the plane instead.

I don't dispute at all that the "unopposed force" makes the plane move, I've said that at least a dozen times. The point that I haven't heard anyone refute is, the movement is all relative to the conveyor belt, which is itself moving relative to the rest of the world.