That plane/conveyor problem - filmed solution
Discussion
the thrust causes the wheels to spin faster and faster. yes, i know they are not directly driven. but this is how the thrust energy is dissipated. it is a linear thrust acting on the airframe and on the landing gear and it results in in an angular moment on the wheels, causing them to accelerate angularly and resulting in circular motion.
because there is very high thrust and the wheels have very little mass (relatively speaking), they spin up to very high speeds. from the conditions stated in the question, the conveyor, always matching the wheel speeds, also spins up to the same high speeds. because the speeds match, the wheels spin, but the plane does not move forward. since it doesn't move forward, there is no airflow past the wings, and it does not experience any lift. the plane does not take off.
just like you pushing on a stroller. it causes the wheels to rotate along the ground. now imagine if you were pushing it on a treadmill. the wheels would rotate, but the stroller would stay in place. now, if you push harder, you would move it forward. BUT, if the treadmill sped up at the same time, then again you would stay put. same thing here.
because there is very high thrust and the wheels have very little mass (relatively speaking), they spin up to very high speeds. from the conditions stated in the question, the conveyor, always matching the wheel speeds, also spins up to the same high speeds. because the speeds match, the wheels spin, but the plane does not move forward. since it doesn't move forward, there is no airflow past the wings, and it does not experience any lift. the plane does not take off.
just like you pushing on a stroller. it causes the wheels to rotate along the ground. now imagine if you were pushing it on a treadmill. the wheels would rotate, but the stroller would stay in place. now, if you push harder, you would move it forward. BUT, if the treadmill sped up at the same time, then again you would stay put. same thing here.
Edited by orgasmicliving!! on Tuesday 12th December 06:21
also, thinking about it logically, you cannot have the wheels match the conveyor belt speed and at the same time move forward over the conveyor belt. either they rotate in place, and the speeds match. or they move over it, in which case the speeds don't match. it's one or the other. it can't be both. my understanding of the question is that it is a "car on a dyno" type of situation and therefore it is a given that the speeds match. so nothing moves forward over the belt.
others think that the question's wording does not imply that the speeds match in that maner, and therefore the plane does move forward (leading to airflow, lift, takeoff).
others think that the question's wording does not imply that the speeds match in that maner, and therefore the plane does move forward (leading to airflow, lift, takeoff).
Edited by orgasmicliving!! on Tuesday 12th December 05:22
Hmmm... I see your point.
I just thought of another approach that I need all the "take-off" supporters to explain:
Imagine you are riding a bicycle with no chain but with a jet engine strapped to your back. You are travelling at 60mph thanks to the thrust from the engine.
What would happen if you:
a) hit a conveyor belt in the road that is doing 80mph in the same direction. You would speed up to 140mph. No question, right?
b) hit a conveyor belt that was doing 80mph in the opposite direction. My guess is that you would end up reversing at 20mph as the thrust from your jet engine is not enough to counter the reversing effects of the conveyor belt.
If this is true, then it perfectly stands to reason to think that if the belt were travelling at 60mph in the oppositer direction, you would come to a complete standstill (imagine the wheels rotating at 60mph on the road and then continue to rotate at the exact same rate when they hit a conveyor belt also doing 60mph). And if the thrust were to increase to give you an equivalent 70mph of movement on a road, and if the speed of the belt were to increase at the same rate, then you wouldn't go anywhere either.
Orgasmic, you know what, I think I am a convert. When I approach it in this way, I think you are right.
I just thought of another approach that I need all the "take-off" supporters to explain:
Imagine you are riding a bicycle with no chain but with a jet engine strapped to your back. You are travelling at 60mph thanks to the thrust from the engine.
What would happen if you:
a) hit a conveyor belt in the road that is doing 80mph in the same direction. You would speed up to 140mph. No question, right?
b) hit a conveyor belt that was doing 80mph in the opposite direction. My guess is that you would end up reversing at 20mph as the thrust from your jet engine is not enough to counter the reversing effects of the conveyor belt.
If this is true, then it perfectly stands to reason to think that if the belt were travelling at 60mph in the oppositer direction, you would come to a complete standstill (imagine the wheels rotating at 60mph on the road and then continue to rotate at the exact same rate when they hit a conveyor belt also doing 60mph). And if the thrust were to increase to give you an equivalent 70mph of movement on a road, and if the speed of the belt were to increase at the same rate, then you wouldn't go anywhere either.
Orgasmic, you know what, I think I am a convert. When I approach it in this way, I think you are right.
a) right
b) right
about wheel speeds and belt speed matching and therefore object not moving on treadmill and then additional thrust resulting in additional acceleration resulting in increased speed being matched by increased conveyor speed resulting in no forward motion again...right again!!!
welcome to the dark side.
PS. aren't you the fellow who likes McLaren F1s? you are right about those too!
b) right
about wheel speeds and belt speed matching and therefore object not moving on treadmill and then additional thrust resulting in additional acceleration resulting in increased speed being matched by increased conveyor speed resulting in no forward motion again...right again!!!
welcome to the dark side.
PS. aren't you the fellow who likes McLaren F1s? you are right about those too!
Edited by orgasmicliving!! on Tuesday 12th December 06:25
-DeaDLocK- said:
Hmmm... I see your point.
I just thought of another approach that I need all the "take-off" supporters to explain:
Imagine you are riding a bicycle with no chain but with a jet engine strapped to your back. You are travelling at 60mph thanks to the thrust from the engine.
What would happen if you:
a) hit a conveyor belt in the road that is doing 80mph in the same direction. You would speed up to 140mph. No question, right?
b) hit a conveyor belt that was doing 80mph in the opposite direction. My guess is that you would end up reversing at 20mph as the thrust from your jet engine is not enough to counter the reversing effects of the conveyor belt.
If this is true, then it perfectly stands to reason to think that if the belt were travelling at 60mph in the oppositer direction, you would come to a complete standstill (imagine the wheels rotating at 60mph on the road and then continue to rotate at the exact same rate when they hit a conveyor belt also doing 60mph). And if the thrust were to increase to give you an equivalent 70mph of movement on a road, and if the speed of the belt were to increase at the same rate, then you wouldn't go anywhere either.
Orgasmic, you know what, I think I am a convert. When I approach it in this way, I think you are right.
I just thought of another approach that I need all the "take-off" supporters to explain:
Imagine you are riding a bicycle with no chain but with a jet engine strapped to your back. You are travelling at 60mph thanks to the thrust from the engine.
What would happen if you:
a) hit a conveyor belt in the road that is doing 80mph in the same direction. You would speed up to 140mph. No question, right?
b) hit a conveyor belt that was doing 80mph in the opposite direction. My guess is that you would end up reversing at 20mph as the thrust from your jet engine is not enough to counter the reversing effects of the conveyor belt.
If this is true, then it perfectly stands to reason to think that if the belt were travelling at 60mph in the oppositer direction, you would come to a complete standstill (imagine the wheels rotating at 60mph on the road and then continue to rotate at the exact same rate when they hit a conveyor belt also doing 60mph). And if the thrust were to increase to give you an equivalent 70mph of movement on a road, and if the speed of the belt were to increase at the same rate, then you wouldn't go anywhere either.
Orgasmic, you know what, I think I am a convert. When I approach it in this way, I think you are right.
a) Wrong - the bike's wheels are on a low friction interface to the frame of the bike. The thrust of the jet engine is being used in countering air resistance - this doesn't change whatever the conveyor's doing. It is the air resistance that limits your speed to 60 mph. The wheels contribute a bit of rolling resistance and bearing friction to overcome, but that's it. You'd end up doing a bit more than 60 relative to a fixed point (due to the slight "speeding up" help from the conveyor), but with your wheels rotating in reverse at a bit less than 20 mph.
b) Wrong - you end up doing a bit less than 60 mph relative to a fixed point (a few mph lost to increased bearing friction/rolling resistance), but with your wheels turning like billy-o underneath you at a bit less than 140 mph.
FFS... 
No, you would speed up to eighty miles an hour. The jet engine is reacting against the static air. You would only accelerate to 140mph if the air mass through which you were travelling also accelerated to 80mph in the same direction.
[Just revisted this last bit with ref to Davi's post above. My assumption here is that you put on the brakes and ride the conveyor to avoid stalling your compressor]
Your wheels would speed up but you would still be doing 60 mph, assuming zero frictional loss. Remember your jet engine reacts against air.
Think of the way hovercraft work. A hovercraft running at 20kts over a 20 kt tide will still only be doing 20kts, regardless of whether it is running with or against the tide.
-DeaDLocK- said:
a) hit a conveyor belt in the road that is doing 80mph in the same direction. You would speed up to 140mph. No question, right?
No, you would speed up to eighty miles an hour. The jet engine is reacting against the static air. You would only accelerate to 140mph if the air mass through which you were travelling also accelerated to 80mph in the same direction.
[Just revisted this last bit with ref to Davi's post above. My assumption here is that you put on the brakes and ride the conveyor to avoid stalling your compressor
] -DeaDLocK- said:
b) hit a conveyor belt that was doing 80mph in the opposite direction. My guess is that you would end up reversing at 20mph as the thrust from your jet engine is not enough to counter the reversing effects of the conveyor belt.
Your wheels would speed up but you would still be doing 60 mph, assuming zero frictional loss. Remember your jet engine reacts against air.
Think of the way hovercraft work. A hovercraft running at 20kts over a 20 kt tide will still only be doing 20kts, regardless of whether it is running with or against the tide.
Edited by Yertis on Tuesday 12th December 08:47
ATG said:
Can you spot what he did there, kids? He said "if the conveyor matches the speed of the wheels". If you make that assumption, he's dead right, the plane remains stationary relative to the ground by definition. If however you say "the conveyor matches the fuselage's speed", then the plane moves and will take off (unless, for example, the tyres explode before they reach twice normal take off speed... which is not an unreasonable scenario). You can make the answer whatever you like by your interpretation of the question.
ATG - as OL, you miss a rather important part of the equation there - in EVERY version of this question ever posted, it states that the "conveyor matches the planes speed" - if you interpret that to mean that the aircraft wheels, then the question is invalid - an aircrafts wheels will only rotate as a result of the aircraft moving forward creating friction against the surface the wheels are rolling on - therefore for the angular velocity of the wheel to be greater than 0, the aircraft must be moving forward.
For the aircraft to be moving forward, the angular velocity of the wheel MUST be greater than the speed of the belt, which in OL's interpretation of the question cannot be the case (conveyor matches wheels speed - cannot happen).
Therefore if you assume that the question relates to the wheel speed, the engines cannot be switched on, the wheels can never move, the conveyor can never move, NOTHING can happen. Which also means the conveyor can never match the speed of the aircraft. So the question doesn't work at all any more, in any way shape or form.
Of course there is also the point that no-one has ever described the speed of a vehicle as an expression of one of it's components angular velocity - primarily because that is not the speed of the vehicle, it's the speed of its component, in which case the question, to infer this, would need to state the name of the component. An aircraft is a bloody great thing with wings, an aircraft wheel is a small bit of metal and rubber attached to the aircraft.
That's me done on it, I got the impression last time that OL was just on a wind up (and I still believe that - it's too obvious to be otherwise) so I've only stated the obvious above to prevent some poor souls becoming confused
orgasmicliving!! said:
the thrust causes the wheels to spin faster and faster. yes, i know they are not directly driven. but this is how the thrust energy is dissipated. it is a linear thrust acting on the airframe and on the landing gear and it results in in an angular moment on the wheels, causing them to accelerate angularly and resulting in circular motion.
because there is very high thrust and the wheels have very little mass (relatively speaking), they spin up to very high speeds. from the conditions stated in the question, the conveyor, always matching the wheel speeds, also spins up to the same high speeds. because the speeds match, the wheels spin, but the plane does not move forward. since it doesn't move forward, there is no airflow past the wings, and it does not experience any lift. the plane does not take off.
just like you pushing on a stroller. it causes the wheels to rotate along the ground. now imagine if you were pushing it on a treadmill. the wheels would rotate, but the stroller would stay in place. now, if you push harder, you would move it forward. BUT, if the treadmill sped up at the same time, then again you would stay put. same thing here.
because there is very high thrust and the wheels have very little mass (relatively speaking), they spin up to very high speeds. from the conditions stated in the question, the conveyor, always matching the wheel speeds, also spins up to the same high speeds. because the speeds match, the wheels spin, but the plane does not move forward. since it doesn't move forward, there is no airflow past the wings, and it does not experience any lift. the plane does not take off.
just like you pushing on a stroller. it causes the wheels to rotate along the ground. now imagine if you were pushing it on a treadmill. the wheels would rotate, but the stroller would stay in place. now, if you push harder, you would move it forward. BUT, if the treadmill sped up at the same time, then again you would stay put. same thing here.
Edited by orgasmicliving!! on Tuesday 12th December 06:21
BOLLOCKS
The thrust DOES NOT MAKE THE WHEELS TURN.
Thrust acts against the air. End of.
The Wheels turn because they are on an axle that allows rotation. Whether it is thrust or a person pushing the plane, the wheels will turn BECAUSE THEY ARE FREE TO DO SO.
Based on your 'premise' if the plane had no wheels it would not move regardless of the magnitude of thrust of the engines.
Sea planes have no wheels but still take off. Odd that.
The only way a plane would stand still in relation to the ground is if it flew into a 160mph headwind. All the wheels do is steer it and keep it of the deck. Wheel speeds and conveyors have nowt to do with anything, though the tyres do have a speed rating much like a car which shouldn't be exceeded.
OL is on a wind up and pretty much admitted such last time. He clearly stated that the answer to the original question is ofcourse it will take off but also said that that question is so easy there's no point in asking it. The wheel speed one is more interesting and as I said earlier is more of a trick logic question than a physics one.
mackie1 said:
The wheel speed one is more interesting and as I said earlier is more of a trick logic question than a physics one.
especially more interesting when you are arguing from that standpoint when everyone else is answering the question as written, eh OL?
Shame it's also just as simple with an even more irrefutable answer...I have not read the whole thread and I'm no physicist but I would have thought that a plane is sucked into the air by the vacuum on it's wing, this is created by the speed of airflow over said wing, now on a belt with the engines at full throttle the object would not be moving and have no air moving over the wing to create the air pressure difference to suck it off as it were.
mackie1 said:
OL is on a wind up and pretty much admitted such last time. He clearly stated that the answer to the original question is ofcourse it will take off but also said that that question is so easy there's no point in asking it. The wheel speed one is more interesting and as I said earlier is more of a trick logic question than a physics one.
agreed, if OL is so inclined perhaps he should send the question to Myth Bustersorgasmicliving!! said:
Sigh. Dead wrong.
If the conveyor belt's speed matches the plane's wheels' speed such that the wheels rotate in place (just like a car on a dyno) then the plane does not move forward relative to the conveyor (just like a car on a dyno). So, no airflow, no lift, no takeoff. This holds true regardless of whether the wheels are directly driven or not. It's just a matter of force. The wheels have a force acting on them, therefore they experience acceleration. This results in a certain speed at any time. If the conveyor matches that speed, then they are not moving with respect to the conveyor. Neither is the plane attached to them. It does not matter that the wheels are directly or indirectly driven. All that matters is the relative speed of the wheels and the conveyor belt.
If the conveyor belt's speed matches the plane's wheels' speed such that the wheels rotate in place (just like a car on a dyno) then the plane does not move forward relative to the conveyor (just like a car on a dyno). So, no airflow, no lift, no takeoff. This holds true regardless of whether the wheels are directly driven or not. It's just a matter of force. The wheels have a force acting on them, therefore they experience acceleration. This results in a certain speed at any time. If the conveyor matches that speed, then they are not moving with respect to the conveyor. Neither is the plane attached to them. It does not matter that the wheels are directly or indirectly driven. All that matters is the relative speed of the wheels and the conveyor belt.
Come on, your winding us up aren't you? Please tell me you are.
They go on about how much congestion might cost the economy in the future, but I fear threads like this cost even more!
I think it's in the governments interests to set up a ministry to answer such questions, so forum users can waste less time arguing in circles and do some work instead
They would conclude that the question gives unsatisfactory criteria to make an accurate assesment of what would happen, and tell us all to get lives for being so simple as to take it seriously!
Dave
I think it's in the governments interests to set up a ministry to answer such questions, so forum users can waste less time arguing in circles and do some work instead
They would conclude that the question gives unsatisfactory criteria to make an accurate assesment of what would happen, and tell us all to get lives for being so simple as to take it seriously!
Dave
apache said:
Yertis said:
Don't they teach physics in schools these days?
Not at OLs school it would seemAh, yes, but you see OL didn't like all the clutter of text books so he digitised them all onto HDD and then threw them out. Unfortunately he then suffered a head crash and got data corruption that by a million-to-one chance made black into white and altered Newton's Laws enough to make him beleive that a conveyor belt could prevent an aircraft from taking off.
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