@kitty-kat
Your analogy made me think of another similar one that *might* help people...
You go to a gym with a horizontal treadmill, you set the treadmill spinning at, lets just say 10kph (or the maximum you think you would ever be able to push a skateboard at normally).
Now, rest the skateboard down on the treadmill and hold it in place, the wheels will be spinning at 10kph but the skateboard will essentially be still, right?
Now get something to hold yourself against so you dont fall off and kill yourself, put one foot on the skateboard and try to hold it there. There will be some force trying to push you in the direction of the treadmill, but not a huge amount.
Now with your other foot on the ground, push yourself, with the skateboard , against the direction of the treadmil.
If you do not fall off from a struggling balancing act, you will move forwards...
In this scenario, you are pushing against a stationary object - the ground.
In the plane scenario, it is also pushing against a stationary object - the air.
You are viewing a low-graphics version of this page. Click the headline to view full version:
Imagine a plane is sitting on a treadmill
A physics and engineering riddle
This:
And this:
Don't match. It's either getting its propulsion from the wheels (1st statement) OR via Newton's third law (2nd). The two are mutually exclusive. One's right, one's wrong. Think about it and make a decision.
QUOTE (BadDoggie @ Dec 19 2006, 12:47 pm) 
...the plane gets its roll via the wheels.
And this:
QUOTE (BadDoggie @ Dec 19 2006, 1:02 pm)
It throws shedloads of air backwards to move itself forwards (Newton's third law), whether via propeller or turbine or even JATO rockets.
Don't match. It's either getting its propulsion from the wheels (1st statement) OR via Newton's third law (2nd). The two are mutually exclusive. One's right, one's wrong. Think about it and make a decision.
When I go to the Tüv they test the brakes by putting the car on rolling cylinders. When the car accelerates the cylinders turn and the car stays 'stationary'. Would this work for a jet plane? What would happen if you put a plane's undercarriage on cylinders and fired up the engine?
PS Fleeting moment of doubt - I'm having trouble getting my head round this one myself.
PS Fleeting moment of doubt - I'm having trouble getting my head round this one myself.
I haven't read most of this, but why would the plane be on a conveyor belt in this scenario if people are still supposed to assume that the plane is moving forwards?
Regardless of the practicalities, isn't this supposed to be a hypothetical question? If you want to take it that literally - has anyone ever seen a conveyor belt that would be big enough for this experiment?
The assumption must be that the plane is moving forwards in relation to the belt, but stationary in relation to the rest of the world...
In that case, I can't see how the plane can take off.
Regardless of the practicalities, isn't this supposed to be a hypothetical question? If you want to take it that literally - has anyone ever seen a conveyor belt that would be big enough for this experiment?
The assumption must be that the plane is moving forwards in relation to the belt, but stationary in relation to the rest of the world...
In that case, I can't see how the plane can take off.
QUOTE (DrivinWest @ Dec 18 2006, 3:28 pm)
Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction. The engines are running at take-off thrust, the brakes are off, etc. Everything is normal save for the fact the plane is on a treadmill.
Can the plane take off?
Can the plane take off?
Right then. Let's understand the bleedin' question before we go off givin' answers.
The conveyor belt is designed to match the speed of the plane exactly, but moves in the opposite direction. The engines are running at take-off thrust, the brakes are off, etc.
Everything is normal save for the fact the plane is on a treadmill.
The plane can't take off because it has no lift. The engines being at take-off thrust only give forward movement cancelled out by the matched conveyor speed in the opposite direction. Under normal conditions there is no lift, because the plane is stationary in air.
EDIT (just for clarification): The thrust would move the air behind the plane, but for every increase in speed, the conveyor matches to bring the plane back to its original position.
What if the belt is travelling at light speed and Captain Jean Luc Picard is on board ? Huh ? What about that ? Fuck the plane and concentrate on my jeans u lot!
What the hell are you talking about Sin.. its been said a million times on this thread that the conveyor will not cancel out the forward movement of the plane. The conveyor cannot bring the plane back to its original position because it cannot exert force on the plane. The only bits of the plane that are touching the conveyor are the wheels, which are free to spin. As long as you don't count friction, the conveyor cannot move the plane in any direction.
Jimbo pointed out a link above, and it explains it perfectly. If you cannot understand what is happening after watching this clip, I would be seriously worried. Here is the link again.
Jimbo pointed out a link above, and it explains it perfectly. If you cannot understand what is happening after watching this clip, I would be seriously worried. Here is the link again.
By the way, this "riddle" is excellent. Good work by DW for posting it. I just gave it to the guys here at work, and they spent 30 minutes arguing about it, two of them correct, and 5 of them not correct. Even when I explained it, the 5 guys were reluctant to believe me.
QUOTE (DrivinWest @ Dec 18 2006, 3:28 pm)
The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction
Does this not mean that the net speed of the plane relative with the ground is 0?
Because otherwise, it's a stupid question and you could leave out the treadmill altogether and rephrase the question as:
QUOTE
A plane is moving forward a bit slower, can it still take off?
You know this is so bloody simple, that after you've realized the answer, you can't imagine how you didn't see it to begin with!
Sin there are three actors in the scenario. The plane, the earth, and the atmosphere. Planes get their propulsion by moving a very small amount of the atmosphere backwards. The plane then moves forward according to Newton's third law. As it moves forward, the stationary atmosphere passes over the wings, providing lift which counteracts the gravity of the earth.
The treadmill is not an actor because it has no effect on the propulsion. Provided the brakes are off the wheels can spin any which way. The plane will move because it is acting on the air, not the treadmill.
The treadmill is not an actor because it has no effect on the propulsion. Provided the brakes are off the wheels can spin any which way. The plane will move because it is acting on the air, not the treadmill.
QUOTE (DrivinWest @ Dec 18 2006, 3:28 pm)
The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction.
So this is the trick part of the question, is it? If the plane moves forwards at factor 10, the conveyor belt moves backwards at factor 10, etc.?
That Link that Hutcho gave is great! Seriously, if you read that link and are still in the "no" camp.. then... I give up lol.
The treadmill does not make the air move, though.
So the Plane is still in relation to the earth and so is the air...
So the Plane is still in relation to the earth and so is the air...
@Wheel. S'ok, I got it now. I know it is all about forward momentum through the air and the lift that provides to the wings. I got caught by the trick.
EDIT: BTW, in my defence, I am a merely laser bloke with the flu. I can't think straight in the laser world with this fuckin' awful headache, so I haven't got a hope in hell with this bollocks. I should have stayed at home and gone to bed.
But, I do have a medical-physics question: How the fuck can this much flourescent green shit come out of my nose when my head is 100 times smaller than the quantity that has already come out?
...and
When is it going to end?
EDIT: BTW, in my defence, I am a merely laser bloke with the flu. I can't think straight in the laser world with this fuckin' awful headache, so I haven't got a hope in hell with this bollocks. I should have stayed at home and gone to bed.
But, I do have a medical-physics question: How the fuck can this much flourescent green shit come out of my nose when my head is 100 times smaller than the quantity that has already come out?
...and
When is it going to end?
QUOTE (Tom17 @ Dec 19 2006, 1:22 pm)
That Link that Hutcho gave is great! Seriously, if you read that link and are still in the "no" camp.. then... I give up lol.
Indeed - essentially it PROVES that a force would still make the plane move, except that the wheels would turn twice as fast. Like I, and others, have been saying. QE fucking D.
Oh - OK, so it's a bit of a trick...
And the plane is still moving forward in relation to the ground
Fair enough
And the plane is still moving forward in relation to the ground
Fair enough
QUOTE (Wheel @ Dec 19 2006, 1:20 pm)
The treadmill is not an actor because it has no effect on the propulsion.
THAT is your mistake. The propulsion of the plane only occurs via the wheels, not from motorisation but via Newton. Engines push backward, plane "wants" to move forward, friction is overcome via wheels. If the wheels are stationary relative to the ground (via the conveyor belt) then there's no forward motion.
QUOTE (Tom17 @ Dec 19 2006, 1:22 pm)
That Link that Hutcho gave is great! Seriously, if you read that link and are still in the "no" camp.. then... I give up
That link makes key mistakes. The upper wheels are pushing against a fixed surface. They're motorised. They take the lower wheels out of the equation. This analogy is then used for a jet engine and it's wrong. A plane isn't pushing against a solid surface. The only way for a jet to transfer its forward motion on the ground is via the wheels. The wheels are being counteracted. No forward motion, no lift, no flight.
woof.
have IQ's dropped here? the treadmill has sod all to do with the plane flying or not. planes fly because the air going over the top of the wings is going faster than the air under the wings and causes lift, there is no drive to the wheels on a plane, the movement comes from the engines blasting air backwards giving thrust, so plane moves forward, treadmill makes absolutely no difference at all unless the pilot has the breaks on
QUOTE (BadDoggie @ Dec 19 2006, 2:29 pm)
The only way for a jet to transfer its forward motion on the ground is via the wheels.
Dude, stop trolling
Im backin the dog here 100%. Some awful freakin morons on this site.
QUOTE (BadDoggie @ Dec 19 2006, 1:29 pm)
The propulsion of the plane only occurs via the wheels, not from motorisation but via Newton.
No, that statement doesn't add up. How does the engine make the wheels turn? Answer: it doesn't.
The plane would move forward even if it was on skis, or floats if it was in the water. The wheels are not providing any propulsion at all. They are a complete red herring.
FOR THE LOVE OF GOD -- THE PLANE WILL TAKE OFF!!!
Im glad the no fly contingency is coming back. The point of the matter is that the plane is not moving, so it can't take off. All this the wheels are freewheelin and so the conveyer belt exerts no power on the plane is BS. The plane is shit ton heavy so of course the treadmill exerts power ont eh plane, albeit less than if the wheels were connected to a drive shaft or a transmission, but still has an effect. Its just like the skate board example, except you put a Ford explorer on top of the skate board and then tell me how easy it is to pusht eh skateboard against the treadmill.
Oh sweet jesus.. I thought our "nearly 500 posts on the subject" was extreme, but over on PhysicsOrgForum the thread has been going on since July 19th 2005 and is currently at PAGE 447! People are still talking about it today. *cries*
We are fracking doomed!!!
We are fracking doomed!!!
QUOTE (parnell @ Dec 19 2006, 1:33 pm)
Im backin the dog here 100%. Some awful freakin morons on this site.
OK, answer this one question then. Look at the diagram below, it is a free-to-roll trolley sat on an uphill conveyor. What happens when you let go of it?
I think I'm going to break something.
Jesus, I can't belive people are still arguing against this - the treadmill makes the wheels turn twice as quick, and that's all. Nothing else matters. Sure, tyres would explode and bearings overheat blah blah fucking blah but the plane WILL fly. DW knows it, I know it, and Sir Isaac Newton knows it.
QUOTE (Tom17 @ Dec 19 2006, 1:39 pm)
Oh sweet jesus.. I thought our "nearly 500 posts on the subject" was extreme, but over on PhysicsOrgForum the thread has been going on since July 19th 2005 and is currently at PAGE 447! People are still talking about it today. *cries*
Oh shit.
QUOTE (sGb27 @ Dec 19 2006, 1:39 pm)
OK, answer this one question then. Look at the diagram below, it is a free-to-roll trolley sat on an uphill conveyor. What happens when you let go of it?
1. It doesnt fly
2. I can't see your hand in the picture
QUOTE (Wheel @ Dec 19 2006, 1:42 pm)
Oh shit.
And just think, people like this are often actually doing real stuff that is important...
QUOTE (Tom17 @ Dec 19 2006, 1:22 pm)
That Link that Hutcho gave is great! Seriously, if you read that link and are still in the "no" camp.. then... I give up lol.
To be fair, Jimbo posted it first. And yes, its an excellent link. People do not have to read 20 odd pages, you just need ot look at this link.
@Hutcho - Being perfectly fair I obtained the link from somebody else too (though I shall allow that person to remain anonymous for now). Suffice to say this person knows their shit and they categorically stated that the plane would fly.
This situation is basically the same as a plane on floaty ski things attempting to take off by heading up a river which is flowing at the same speed the plane is going. There is some friction between the water and the skis, but the plane would still move upstream and take off. Correct?
I think several people are just trolling now to get it up to 500
Sorry about that. So many pages, I just noticed it when you posted it. Sorry Jimbo
edit: None of the "no fly"ers commented oon the skateboard+treadmill scenario. Other than putting a car on it which is errr.. a tad heavier than gym treadmills and skateboards are designed to hold up
edit: None of the "no fly"ers commented oon the skateboard+treadmill scenario. Other than putting a car on it which is errr.. a tad heavier than gym treadmills and skateboards are designed to hold up
QUOTE (sGb27 @ Dec 19 2006, 1:39 pm)
OK, answer this one question then. Look at the diagram below, it is a free-to-roll trolley sat on an uphill conveyor. What happens when you let go of it?
Well, it depends, if the premise is that the buggy and the conveyer belt are to move at the same speed. Then it stays there. If you could get the conveyer going fast enough the cart could go up hill. But the cart could also slide right down.
However in our question, it is 1) flat and 2) stipulated that the conveyer goes the same speed as the plane.
@Dolfan - you're kidding right? The trolley stays put? No chance - it'll roll down, wheels spinning like crazy - the conveyor can move at an equal or even GREATER speed uphill, but it'll slide down. Why? Coz the wheels can spin either way, therefore the conveyor can't exert its full force on the buggy, whilst gravity can (gravity doesn't drive the wheels remember - it acts independently of the wheels, just like the jet's thrust) - hence it rolls down hill.
QUOTE (cantthinkofawittyname @ Dec 19 2006, 1:47 pm)
This situation is basically the same as a plane on floaty ski things attempting to take off by heading up a river which is flowing at the same speed the plane is going. There is some friction between the water and the skis, but the plane would still move upstream and take off. Correct?
No, because in the above example, if the propeller stopped, the plane would float downstream. In this case, the plane has to actively fight against the water.
As far as I am concerned, with the treadmill example, assuming a perfect system, if the plane stopped it's engines, it would not move anywhere, since the reverse movement of the treadmill would only cause the plane's wheels to spin, but not actually move the plane..
It's not a perfect system tho, the plane would move back because of friction between the wheels and the plane. If there was no friction between the water and the planes skis, it wouldn't float downstream
QUOTE (dolfan @ Dec 19 2006, 1:49 pm)
Well, it depends, if the premise is that the buggy and the conveyer belt are to move at the same speed. Then it stays there. If you could get the conveyer going fast enough the cart could go up hill.
Oh really, and how exactly does the conveyor exert a force on the buggy? Have you ever tried to tried to pull anything with wheels towards you by pulling what it's resting on? It doesn't work unless you do it really slowly.
QUOTE (dolfan @ Dec 19 2006, 1:49 pm)
But the cart could also slide right down.
Which it will, try it next time you are at the airport.
QUOTE (dolfan @ Dec 19 2006, 1:49 pm)
However in our question, it is 1) flat
Tilt your head a bit until the conveyor is horizontal, the sideways gravity component is just like the force from the jet on the plane.
QUOTE (dolfan @ Dec 19 2006, 1:49 pm)
and 2) stipulated that the conveyer goes the same speed as the plane.
Indeed it is, so what's to stop the plane going 200mph in one direction while the conveyor is going 200mph in the other direction? Note that DOES NOT mean the plane is stationary, it merely means the plane wheels would be spinning at 400mph.
@ cantthinkofawittyname
Airplanes have been moved by people. Once they get moving they keep moving. So the resistance is not very high.
Airplanes have been moved by people. Once they get moving they keep moving. So the resistance is not very high.
No i am not kiidding. The conveyer could certainly move fast enough to push the buggy back. It would have to go very, very fast, but it is certainly possible.
Think of water going down a slope like this. At some speed the water goes down, increase x and it stays put, increase y and it travels up hill.
Just because we are talking about free wheels, doesn't mean we are talking about resistance free wheels. There will be force exerted on the buggy/plane whatever by the conveyer.
Think of water going down a slope like this. At some speed the water goes down, increase x and it stays put, increase y and it travels up hill.
Just because we are talking about free wheels, doesn't mean we are talking about resistance free wheels. There will be force exerted on the buggy/plane whatever by the conveyer.
The treadmill only goes as fast as the wheels - not 'very very fast'.
The. Only. Plane. That. Won't. Fly. Is. One. That. Is. Propelled. By. It's. WHEELS. (which planes AREN'T!)
QUOTE (Wheel @ Dec 19 2006, 2:05 pm)
The treadmill only goes as fast as the wheels - not 'very very fast'.
Let me clarify, no stipulations, the conveyer and the cart are free to move as fst as they want, no need to move relative to each other. the conveyer could go fast enough to push the cart up hill.
QUOTE (dolfan @ Dec 19 2006, 2:04 pm)
Just because we are talking about free wheels, doesn't mean we are talking about resistance free wheels. There will be force exerted on the buggy/plane whatever by the conveyer.
Right, we're slowly getting there now. The only force the conveyor can transmit to the plane is due to the friction in the wheels, looks like you got that one. So for the plane to remain stationary this would need to match the force from the engines, right? But, the rolling resistance of a plane is "only" like a 1000 N or so (a very strong person can pull a plane). Go look up how much thrust a plane generates, it is a tad bigger than 1000 N.
You must have done this, at a supermarket or airport: the trolleys have locks [brakes] to stop them rolling backwards. Otherwise it would be impossible to put a trolley on a rolling road type escalator.
I think I have solved it. The "fly" people and the "no fly" people are each making an unannounced assumption about the question...
Now, that is not clear if it is the groundspeed of the plane or the speed of the plane relative to the treadmill itself.
If you go for the more logical(in my mind) of those 2 guess-assumptions, you will have the speed of the treadmill matching the groundspeed of the plane. The plane will take off as all the "will fly" people have been saying.
If however you say...
... then you will get a utterly impossible scenario as desribed below.. This kinda ties in with sgb27's calculations about the belt having to accelerate at about 40G to stop the plane from moving...
I personally think the question would be better worded with "groundspeed" in it to make things a bit clearer.
QUOTE (DrivinWest @ Dec 18 2006, 3:28 pm)
Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction. The engines are running at take-off thrust, the brakes are off, etc. Everything is normal save for the fact the plane is on a treadmill.
Now, that is not clear if it is the groundspeed of the plane or the speed of the plane relative to the treadmill itself.
If you go for the more logical(in my mind) of those 2 guess-assumptions, you will have the speed of the treadmill matching the groundspeed of the plane. The plane will take off as all the "will fly" people have been saying.
If however you say...
QUOTE (DrivinWest @ Dec 18 2006, 3:28 pm)
Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyor belt is designed to match the speed of the plane relative to the treadmill itself but moves in the opposite direction. The engines are running at take-off thrust, the brakes are off, etc. Everything is normal save for the fact the plane is on a treadmill.
... then you will get a utterly impossible scenario as desribed below.. This kinda ties in with sgb27's calculations about the belt having to accelerate at about 40G to stop the plane from moving...
QUOTE
Yes, it will take off. However not because of forward movement. As
with many hypothetical questions we must assume some things not stated
in the question. Let's assume this is a magical converor belt that
can run fast enough to keep the aircraft relatively stationary with
respect to the ground next to the converor belt. We must also assume
that the aircraft tires/wheels will not fly apart (which they likely
would). Further, let's assume the intent of the question is that this
converor belt has the ability to counter the forward motion of the
aircraft. Since the aircraft will tend to move, not as a result of
it's wheels roating, but as a result of engine thrust, the only way
this magic converor belt can "stop" the aircraft from moving forward
is to accelearte so fast that the aircraft engine's energy is all
consumed by wheel friction and the initeria of spinning up the
aircrafts wheels. Very quickly the converor belt will be moving at
impossible speeds, but this is a hypothetical question and if we
assume it is possible for a converor belt to hold this aircraft in
place we have to assume it can move this fast.
So, very quickly after the aircraft attempts to move forward, the
conveyor belt is spinning under the aircraft at thousands of feet per
second. Friction between the air at the surface of the converor belt
and the belt will cause the air to move with respect to the ground
next to the belt. At some point the magic belt will be moving fast
enough to generate enough airflow over the wings that the aircraft
will lift off the belt, again assuming the belt and wheels/tires are
magic and do not fly apart.
So, yes the aircraft will lift off the belt. What happens next is the
subject of another debate.
with many hypothetical questions we must assume some things not stated
in the question. Let's assume this is a magical converor belt that
can run fast enough to keep the aircraft relatively stationary with
respect to the ground next to the converor belt. We must also assume
that the aircraft tires/wheels will not fly apart (which they likely
would). Further, let's assume the intent of the question is that this
converor belt has the ability to counter the forward motion of the
aircraft. Since the aircraft will tend to move, not as a result of
it's wheels roating, but as a result of engine thrust, the only way
this magic converor belt can "stop" the aircraft from moving forward
is to accelearte so fast that the aircraft engine's energy is all
consumed by wheel friction and the initeria of spinning up the
aircrafts wheels. Very quickly the converor belt will be moving at
impossible speeds, but this is a hypothetical question and if we
assume it is possible for a converor belt to hold this aircraft in
place we have to assume it can move this fast.
So, very quickly after the aircraft attempts to move forward, the
conveyor belt is spinning under the aircraft at thousands of feet per
second. Friction between the air at the surface of the converor belt
and the belt will cause the air to move with respect to the ground
next to the belt. At some point the magic belt will be moving fast
enough to generate enough airflow over the wings that the aircraft
will lift off the belt, again assuming the belt and wheels/tires are
magic and do not fly apart.
So, yes the aircraft will lift off the belt. What happens next is the
subject of another debate.
I personally think the question would be better worded with "groundspeed" in it to make things a bit clearer.
@ Wheel
I'm not saying it would move back as fast as the treadmill is moving, no way near, just slowly
I'm not saying it would move back as fast as the treadmill is moving, no way near, just slowly
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