Can the airplane take off?

[uberman]

Originally posted by: beansbaxter
Alright, this is NOT a homework question. In another thread on the Anandtech forums, I saw this mentioned deep within a thread but it was never addressed. I am still curious so I ask it here:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse. Can the airplane take off?

It wasn't really clear and was misleading because he did not post the stall speed of the plane.

What is the stall speed of the plane?

 

[BrownTown]

just a sort of "hypothetical" question, but if I start curssing alot will that get this thread closed, or simply get my post deleted and me banned? I'm trying to find someone of getting this idiotic thread closed, but I'm not sure how to do it without getting banned?

Anyone want to randomly get in a flame war full of personal attacks in order to get this thread closed.

Ill start:

Dear <insert name here>, your mother is fat, and furthermore I question your sexuality.

 

[smack Down]

Originally posted by: Smilin

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

 

[sao123]

Originally posted by: uberman

Originally posted by: beansbaxter
Alright, this is NOT a homework question. In another thread on the Anandtech forums, I saw this mentioned deep within a thread but it was never addressed. I am still curious so I ask it here:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse. Can the airplane take off?

It wasn't really clear and was misleading because he did not post the stall speed of the plane.

What is the stall speed of the plane?

the stall speed of the plane is pointless... The treadmill does not slow the the plane by even 1 mile per hour of what it would be on a normal runway.
once again... put on some skates, tie a rope onto the wall, and pull yourself to the wall.
Now repeat the process on your motorized excercise treadmill.
Voila... you've moved forward at the same speed as on the ground, only your wheels moved faster, because they had to travel further in the same amount of time.


If we killed all the people who believe that the plane woudl stay stationary... would that be moronocide?

 

[Gibsons]

Originally posted by: smack Down

Originally posted by: Smilin

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

If the plane doesn't move, then its velocity is zero and therefore so is the treadmills.

 

[uberman]

Originally posted by: BrownTown
just a sort of "hypothetical" question, but if I start curssing alot will that get this thread closed, or simply get my post deleted and me banned? I'm trying to find someone of getting this idiotic thread closed, but I'm not sure how to do it without getting banned?

Anyone want to randomly get in a flame war full of personal attacks in order to get this thread closed.

Ill start:

Dear <insert name here>, your mother is fat, and furthermore I question your sexuality.

I take personal offense in your reference to my family members. In other words, put a sock in it.

 

[sao123]

Originally posted by: smack Down

Originally posted by: Smilin

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

the definition of the problem is a paradox... it cant exist.

 

[Smilin]

Originally posted by: smack Down

Originally posted by: Smilin

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

*sigh*

We're coming back around to that 1) you are trolling or 2) you are an idiot thing. You're gonna end up getting flamed and likely getting the thread closed down.

Imagine a treadmill the length of an entire runway. Assume takeoff speed for the plane is 100knots.

Now assume we have three "speedometers".
1) The Pitot on the aircraft nose that measures airspeed.
2) A traditional speedometer, just like a car, that is connected to the aircraft wheel.
3) A traditional speedometer connected to one of the rollers on the treadmill.

Now imagine this:
Plan fires up the engines and applies thrust.
The plane accelerates to 100knots of airspeed per the Pitot (speedometer #1).
If it was a normal runway the speedometer #2 on the wheel would also show 100knots.

Now imagine the treadmill is moving backwards at the same speed that the plane is moving forward.
speedometer #1 (airspeed, pitot) shows +100knots.
speedometer #3 (on treadmill) shows -100knots (100 knots backwards).
speedometer #2 (plane wheels) shows +200knots (wheel moving forward through the air at 100, while rolling over surface goign backwards at 100 = spinning at 200.

PLANE WOULD TAKE OFF BECAUSE THE AIRSPEED IS 100 KNOTS.

Now if you spin the treadmill at 100knots backwards and LOCK THE BRAKES on the planes wheels you'll get this:
speedometer #1 (airspeed, pitot) shows -100knots (100 knots backwards).
speedometer #3 (treadmill) shows -100knots (100 backwards)
speedometer #2 (plane wheels) shows 0 knots...the wheels are locked so the plane goes at whatever speed the treadmill is going.

The plane would not take off because it's going 100knots backwards (it would likely flip all over the place and fall off the treadmill)

Now if you spin the treadmill FORWARD at 100 knots while the planes engines are driving it forward:
speedometer #1 (airspeed) = 100 knots.
speedometer #3 (treadmill) = 100 knots.
speedometer #2 (wheels) = 0 knots (the ground/treadmill is moving at same speed as plane's propeller is moving it forward).

Now if you spin the treadmill FORWARD at 100 knots and lock the brakes on the plane you'll get:
#1 (airspeed) = 100
#3 (treadmill) = 100
#2 (wheels) = 0 (same as before, but now you've allowed the airplane to take off without using it's own engine for propulsion...it will of course slow down and stop flying very shortly thereafter).


That's it dude. That's as deep as the explanation gets. Unless you are wearing a lead helmet this should get through.

 

[Thraxen]

Originally posted by: sao123
the definition of the problem is a paradox... it cant exist.

Are you saying that the speed of the treadmill can't match the speed of the plane as given in the problem? Sure it can. I guess it depends on how you read the problem, but I read it as the speed of the belt on the treadmill matches the overall forward motion of the plane (i.e. the entire plane, not the speed the wheels are turning). This is possible and the most logical, so that's what I assumed it meant.

 

[Dulanic]

Originally posted by: Thraxen

Originally posted by: sao123
the definition of the problem is a paradox... it cant exist.

Are you saying that the speed of the treadmill can't match the speed of the plane as given in the problem? Sure it can. I guess it depends on how you read the problem, but I read it as the speed of the belt on the treadmill matches the overall forward motion of the plane (i.e. the entire plane, not the speed the wheels are turning). This is possible and the most logical, so that's what I assumed it meant.

Here is why it is a paradox.... the engines use the air for thrust, the treadmill would never be able to create any reasonable amount of force to move the airplane back. The most force it could create would be the resistance of the bearings in the wheels... You could make the treadmill go 1000 MPH backwards and even with the engines off it would take quite some time to get the plane moving fast backwards. So the reason it is a paradox... No matter what you did with the treadmill it could NEVER create enough force to keep the plane from moving foward.... the only end result would be burnt out bearings from moving so fast.

The treadmill would have no noticable result no matter what it did except causing damage to the wheels... end of story.

 

[brikis98]

Originally posted by: Dulanic

Originally posted by: Thraxen

Originally posted by: sao123
the definition of the problem is a paradox... it cant exist.

Are you saying that the speed of the treadmill can't match the speed of the plane as given in the problem? Sure it can. I guess it depends on how you read the problem, but I read it as the speed of the belt on the treadmill matches the overall forward motion of the plane (i.e. the entire plane, not the speed the wheels are turning). This is possible and the most logical, so that's what I assumed it meant.

Here is why it is a paradox.... the engines use the air for thrust, the treadmill would never be able to create any reasonable amount of force to move the airplane back. The most force it could create would be the resistance of the bearings in the wheels... You could make the treadmill go 1000 MPH backwards and even with the engines off it would take quite some time to get the plane moving fast backwards. So the reason it is a paradox... No matter what you did with the treadmill it could NEVER create enough force to keep the plane from moving foward.... the only end result would be burnt out bearings from moving so fast.

The treadmill would have no noticable result no matter what it did except causing damage to the wheels... end of story.

you're exactly right Dulanic, except there is no paradox and your reasoning is exactly why the plane will take off.

formulation of the problem:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse.

it does NOT say that the treadmill is keeping the plane stationary, that is just something you (incorrectly) assumed. in fact, as you pointed it, it's virtually impossible in real life for that to be the case. instead, the problem says that if the plane is moving forward at X mph, the treadmill is moving backwards at X mph.

as you noticed yourself, this DOES NOT keep the plane stationary. so there is no paradox. the plane accelerates, the wings generate lift and it takes off.

anyone who comes to the conclusion that the plane doesn't take off is only doing so because they ASSUME the plane is stationary, which the problem does not state and you can NOT reasonably interpret that to be the meaning.

 

[gsellis]

The morons are strong in this thread... Who cares what the speed of the treadmill is. In fact, I will let you DOUBLE the treadmill speed to that of the airplane on the treadmill. It will still take off. I will allow complete tire failure. If I have a 777, full of psgrs, it will take off. There will be lots of tire smoke and sparks, but it will take off.

And lift is about the speed differential over the surface where one is faster than the other. The suck and the blow move it, which makes it suck = flight.

 

[sao123]

Originally posted by: brikis98

Originally posted by: Dulanic

Originally posted by: Thraxen

Originally posted by: sao123
the definition of the problem is a paradox... it cant exist.

Are you saying that the speed of the treadmill can't match the speed of the plane as given in the problem? Sure it can. I guess it depends on how you read the problem, but I read it as the speed of the belt on the treadmill matches the overall forward motion of the plane (i.e. the entire plane, not the speed the wheels are turning). This is possible and the most logical, so that's what I assumed it meant.

Here is why it is a paradox.... the engines use the air for thrust, the treadmill would never be able to create any reasonable amount of force to move the airplane back. The most force it could create would be the resistance of the bearings in the wheels... You could make the treadmill go 1000 MPH backwards and even with the engines off it would take quite some time to get the plane moving fast backwards. So the reason it is a paradox... No matter what you did with the treadmill it could NEVER create enough force to keep the plane from moving foward.... the only end result would be burnt out bearings from moving so fast.

The treadmill would have no noticable result no matter what it did except causing damage to the wheels... end of story.

you're exactly right Dulanic, except there is no paradox and your reasoning is exactly why the plane will take off.

formulation of the problem:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse.

it does NOT say that the treadmill is keeping the plane stationary, that is just something you (incorrectly) assumed. in fact, as you pointed it, it's virtually impossible in real life for that to be the case. instead, the problem says that if the plane is moving forward at X mph, the treadmill is moving backwards at X mph.

as you noticed yourself, this DOES NOT keep the plane stationary. so there is no paradox. the plane accelerates, the wings generate lift and it takes off.

anyone who comes to the conclusion that the plane doesn't take off is only doing so because they ASSUME the plane is stationary, which the problem does not state and you can NOT reasonably interpret that to be the meaning.

when I said there is a paradox... what smack Down is saying is that the problem states that the treadmill hold the plane stationary...

Originally posted by: smack Down
IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

which makes the way he interprets the question a paradox, because it cant ever happen.

READING COMPREHENSION FTW.

 

[Netopia]

I only read the first page, but I'll try my hand at this. I seriously can't believe that ANYONE would think the plane would ever take off.

In the original question, the OP said that the treadmill would match the speed of the plane, only in reverse.

Let's assume that the plane needed 100MPH of air speed over the wings to get enough lift to become airborne.

Let's assume a 0 MPH relative wind speed.

If the plane generates enough thrust to move forward at 1 MPH, but the belt is going backward 1 MPH, the plane still has zero relative wind speed over the wings.

At enough thrust to move forward at 10 MPH, the belt is moving backward at 10 MPH and the plane still has a relative wind speed of ZERO over the wings.

At enough thrust to move forward at a MILLION miles per hour, the belt is moving backward at a million miles per hour and there is STILL no air flow over the wings to provide ANY lift!

Now, if a 100 MPH gust of head wind came out of nowhere, it's possible that the plane would INSTANTLY be airborne and be able to move forward because it already had enough thrust to keep it airborne ONCE ALOFT. But without air to create lift in the first place, it can never leave the conveyor belt.

If a boat was capable of going 10 MPH maximum and you headed it upstream in a river flowing at 10 MPH, do you think you'd move?

Maybe if we change venues it will be easier for some of you.... we have a runway on the Moon that is perfectly flat and goes all the way around the moon like a ring, there is no conveyor, and you have a normally shaped (but air tight) air liner, except that it has rocket engines that can fire for years on end. Will the plane ever get airborne? If you say "yes", you're an idiot. If you say no, you have your answer to why the one on earth wouldn't lift... YOU HAVE TO HAVE AIRSPEED FOR LIFT!

Joe

 

[brikis98]

Originally posted by: sao123
when I said there is a paradox... what smack Down is saying is that the problem states that the treadmill hold the plane stationary...

Originally posted by: smack Down
IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

which makes the way he interprets the question a paradox, because it cant ever happen.

READING COMPREHENSION FTW.

well, if your original post had said "smack Down's definition of the problem is a paradox" (or if Dulanic's post had explained why smack Down's version would be paradoxical), your posts would've made sense. as written, it definitely seemed to me like you guys were saying the original problem contained a paradox.

at any rate, the important thing is that we agree (i think): the plane takes off and smack down is a troll and/or stubborn idiot. any other interpretation is just flat out unreasonable

 

[MrPickins]

Originally posted by: Netopia
I only read the first page, but I'll try my hand at this. I seriously can't believe that ANYONE would think the plane would ever take off.

In the original question, the OP said that the treadmill would match the speed of the plane, only in reverse.

Let's assume that the plane needed 100MPH of air speed over the wings to get enough lift to become airborne.

Let's assume a 0 MPH relative wind speed.

If the plane generates enough thrust to move forward at 1 MPH, but the belt is going backward 1 MPH, the plane still has zero relative wind speed over the wings.

At enough thrust to move forward at 10 MPH, the belt is moving backward at 10 MPH and the plane still has a relative wind speed of ZERO over the wings.

At enough thrust to move forward at a MILLION miles per hour, the belt is moving backward at a million miles per hour and there is STILL no air flow over the wings to provide ANY lift!

Now, if a 100 MPH gust of head wind came out of nowhere, it's possible that the plane would INSTANTLY be airborne and be able to move forward because it already had enough thrust to keep it airborne ONCE ALOFT. But without air to create lift in the first place, it can never leave the conveyor belt.

If a boat was capable of going 10 MPH maximum and you headed it upstream in a river flowing at 10 MPH, do you think you'd move?

Maybe if we change venues it will be easier for some of you.... we have a runway on the Moon that is perfectly flat and goes all the way around the moon like a ring, there is no conveyor, and you have a normally shaped (but air tight) air liner, except that it has rocket engines that can fire for years on end. Will the plane ever get airborne? If you say "yes", you're an idiot. If you say no, you have your answer to why the one on earth wouldn't lift... YOU HAVE TO HAVE AIRSPEED FOR LIFT!

Joe

Next time you should read the whole thread,

 

[sao123]

Originally posted by: sao123

Originally posted by: smack Down

Originally posted by: Smilin

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

the definition of the problem is a paradox... it cant exist.

Originally posted by: brikis98

well, if your original post had said "smack Down's definition of the problem is a paradox" (or if Dulanic's post had explained why smack Down's version would be paradoxical), your posts would've made sense. as written, it definitely seemed to me like you guys were saying the original problem contained a paradox.

at any rate, the important thing is that we agree (i think): the plane takes off and smack down is a troll and/or stubborn idiot. any other interpretation is just flat out unreasonable

I quoted the guys post.... Ill even bold it for you...

 

[brikis98]

Originally posted by: Netopia
I only read the first page, but I'll try my hand at this. I seriously can't believe that ANYONE would think the plane would ever take off.

In the original question, the OP said that the treadmill would match the speed of the plane, only in reverse.

Let's assume that the plane needed 100MPH of air speed over the wings to get enough lift to become airborne.

Let's assume a 0 MPH relative wind speed.

If the plane generates enough thrust to move forward at 1 MPH, but the belt is going backward 1 MPH, the plane still has zero relative wind speed over the wings.

At enough thrust to move forward at 10 MPH, the belt is moving backward at 10 MPH and the plane still has a relative wind speed of ZERO over the wings.

At enough thrust to move forward at a MILLION miles per hour, the belt is moving backward at a million miles per hour and there is STILL no air flow over the wings to provide ANY lift!

Now, if a 100 MPH gust of head wind came out of nowhere, it's possible that the plane would INSTANTLY be airborne and be able to move forward because it already had enough thrust to keep it airborne ONCE ALOFT. But without air to create lift in the first place, it can never leave the conveyor belt.

If a boat was capable of going 10 MPH maximum and you headed it upstream in a river flowing at 10 MPH, do you think you'd move?

Maybe if we change venues it will be easier for some of you.... we have a runway on the Moon that is perfectly flat and goes all the way around the moon like a ring, there is no conveyor, and you have a normally shaped (but air tight) air liner, except that it has rocket engines that can fire for years on end. Will the plane ever get airborne? If you say "yes", you're an idiot. If you say no, you have your answer to why the one on earth wouldn't lift... YOU HAVE TO HAVE AIRSPEED FOR LIFT!

Joe

seriously, you look pretty foolish right now and you need to read the rest of this thread.

 

[Netopia]

when I said there is a paradox... what smack Down is saying is that the problem states that the treadmill hold the plane stationary...

quote:
Originally posted by: smack Down
IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.



which makes the way he interprets the question a paradox, because it cant ever happen.

READING COMPREHENSION FTW.

It's not a paradox. Let's say that the plane moves forward 1mm, and then the belt adjusts and the plane isn't moving forward any longer... and then the plane moves forward 1mm and the belt adjusts... and so on. The only forward movement is that 1 mm and so for 1mm there is speed, but that speed is instantly negated by the matching of speed by the belt. Unless the belt were infinitely long, eventually the plane would simply move the last 1mm forward at some point and the nose would fall off of the end.

I prefer to think of the question (and this is NOT what the OP asked) as a belt that precisely matches the speed of the plane so that the plane never achieves any forward movement. This eliminates even the minuscule movement that might mislead some people into believing that the plane would eventually take off.

Perhaps another way to look at this would be to ask:

If there was a plane that had a mile long chain attached to it, but the chain was taught and there was zero slack... if that plane put its engines on full, at some point would it lift off of the ground and hover like a kite? Unless, perhaps there were rockets involved that created their own lift, the plane would never lift ... no wind over the wings.

 

[Netopia]

Hmmm.... I can't see where I went wrong... gimme a hint, which page? How far down? How can the plane ever lift?

Joe

 

[acole1]

I think the point made by some above is that the plane can (and will) move forward 1MPH even while the belt is moving back 10MPH. There is nothing pulling the plane back. The belt exerts no negative force upon the plane since the wheels are free spinning.

The plane thrusts forward on the air, and the wheels spin freely underneath it as fast as they want to.

The faster the belt moves the faster the wheels move backwards, but with no effect on the plane.

 

[brikis98]

Originally posted by: sao123

Originally posted by: sao123

Originally posted by: smack Down

IF you are moving forward on a treadmill then the treadmill is no longer matching the speed of the object relitive to the treadmill. So by definition of how you read the problem (if you read it one way) the plane can't move.

the definition of the problem is a paradox... it cant exist.

I quoted the guys post.... Ill even bold it for you...

as i said already, i agree with you, but your post is still ambiguous. it's hard to tell if you're arguing that smack down's interpretation of the problem is paradoxical or that whatever he's saying is irrelevant because the original problem is paradoxical. you didn't qualify which (or who's) "definition of the problem" you are referring to, even if he is being quoted there. now that you've explained it, it makes sense and i agree with you, but without that explanation, it's unclear.

 

[brikis98]

Originally posted by: Netopia
Hmmm.... I can't see where I went wrong... gimme a hint, which page? How far down? How can the plane ever lift?

Joe

the gist is that an airplane != a car. the airplane's wheels are not connected to any drive train. the treadmill spinning under the wheels of the airplane, even with the engines off, will NOT move the airplane backwards (well, realistically due to some friction/torque it prob. will, but only a tiny amount).

the engines of an airplane push against the air around it to propel. it does NOT push off the ground like a car. therefore, the treadmill is virtually irrelevant and the airplane will build speed and move down the treadmill as if it wasn't even there. the only difference, again, aside from the small amount of torque/friction on the wheels, is that the wheels are spinning much faster.

 

[Netopia]

Originally posted by: acole1
I think the point made by some above is that the plane can (and will) move forward 1MPH even while the belt is moving back 10MPH. There is nothing pulling the plane back. The belt exerts no negative force upon the plane since the wheels are free spinning.

The plane thrusts forward on the air, and the wheels spin freely underneath it as fast as they want to.

The faster the belt moves the faster the wheels move backwards, but with no effect on the plane.

Perhaps I just can't let go of reality. According to what you just laid out, if the plane was simply sitting idle and someone turned the conveyor on at 10,000 MPH in reverse, the plane would simply sit there because the tires would move independently of the plane, is that right? I guess in some hypothetical world this would work, but I don't see if working in reality.

Joe

 

[MrPickins]

oopsie