Right,
We are up to 540 replies on the other forum 😀 - it's here http://www.visordown.com/forums/showthread.php?t=235105
It's not quite as civilised as this place so don't click the link if you will be offended by the odd swear word or naughty avatar - or if you are under 18 🙂
We are up to 540 replies on the other forum 😀 - it's here http://www.visordown.com/forums/showthread.php?t=235105
It's not quite as civilised as this place so don't click the link if you will be offended by the odd swear word or naughty avatar - or if you are under 18 🙂
maxro said:
Actually I think you will be surprised at the result. My prediction is that the skateboard will remain stationary, or possibly even go backwards... backwards you say?? the surface of a travelator is generally rougher than the floor (I'm assuming smooth surface on the floor) and there will likely be more rolling resistance, so the amount of thrust required to make it move along the floor, won't be enough to counteract the backwards movement on the travelator as there is more rolling resistance and hence a bigger loss.
What happens if you turn off the fan, the skateboard goes backwards right????
or how about you just drop the skateboard onto the travelator with no fan, will it's wheels start spinning and it will stay stationary?? not likely, it will of course go backwards.
my only doubt here is whether or not the amount of force required to overcome the backwards movement of the travelator is less because the wheel can roll. If the force is less, then it can take off, if it isn't then it can't. I'm afraid my physics is too rusty to make my mind up one way or the other, but my gut feeling is it can't.
quickshift's question about the plane landing on a travelator is an interesting one though quite different as there is a significant amount of kinetic energy in both the plane and the belt meeting at one instant, whereas the original question is starting with no kinetic energy...., perhaps someone with a belt sander a variac and a matchbox car (and no regard for safety) could do an experiment 😉 though having the matchbox car fly through the air at the correct speed and land on the precily calibrated belt sander might be difficult.... perhaps a belt sander running full pelt might prove more telling as my guess is the car wouldn't roll off the front, but would in fact fly off backward at great velocicty 😉
Tony.
OK I've changed my mind 😉 was thinking about wearing rollerskates on a moving travelator.....
if the travelator moves on a flat surface you will move with it... if the travelator is going up an incline you will stay at the bottom of the incline, no matter how fast the travelator spins (ingnoring bearing failure) you will stay at the bottom of the travelator. Gravity is holding you there, and gravity is constant. therefore the force required to overcome the movement of the travelator (when the object is on wheels) is less than or equal to the force of gravity....... I suspect a lot less). If anyone is in Sydney and has a pair of rollerskates, there is a suitable inclined travelator at the domain car park 😉
Sooooo back to the plane, once the initial force required to counteract the conveyors pulling force, which by the above reasoning should be constant (regardless of the conveyors speed), is overcome, the plane will start to accelerate forward (with respect to the surroundings and air) and it will take off 🙂
Tony.
if the travelator moves on a flat surface you will move with it... if the travelator is going up an incline you will stay at the bottom of the incline, no matter how fast the travelator spins (ingnoring bearing failure) you will stay at the bottom of the travelator. Gravity is holding you there, and gravity is constant. therefore the force required to overcome the movement of the travelator (when the object is on wheels) is less than or equal to the force of gravity....... I suspect a lot less). If anyone is in Sydney and has a pair of rollerskates, there is a suitable inclined travelator at the domain car park 😉
Sooooo back to the plane, once the initial force required to counteract the conveyors pulling force, which by the above reasoning should be constant (regardless of the conveyors speed), is overcome, the plane will start to accelerate forward (with respect to the surroundings and air) and it will take off 🙂
Tony.
I think some of us have the impression that the bearings in the wheels are either glued, welded or seized. The bearings decouple the plane from the effects of the wheels rolling or not (with a small error).
Landing or taking off makes no difference to the question at all. Thrust does not act on the wheels directly. The air mass is extremely loosely coupled to the belt and that has no major effect on the air mass.
If the plane does not take off, there is a problem with Einstein's theories of relativity. The normal force applied to the wheels by the planes weight do not couple the plane to the belt, or earth. The bearings in the wheels were designed to roll under this combination of forces. If this were not the case, flight would not be possible using this technology. Cars and trains wouldn't work either. The wheel was a great invention, pivotal even.
eVITAERC, reread and rethink. The main questions are: what is coupled and what is decoupled, and where are the forces applied. Eliminate everything that is decoupled and look at the forces applied, and to what masses.
-Chris
Landing or taking off makes no difference to the question at all. Thrust does not act on the wheels directly. The air mass is extremely loosely coupled to the belt and that has no major effect on the air mass.
If the plane does not take off, there is a problem with Einstein's theories of relativity. The normal force applied to the wheels by the planes weight do not couple the plane to the belt, or earth. The bearings in the wheels were designed to roll under this combination of forces. If this were not the case, flight would not be possible using this technology. Cars and trains wouldn't work either. The wheel was a great invention, pivotal even.
eVITAERC, reread and rethink. The main questions are: what is coupled and what is decoupled, and where are the forces applied. Eliminate everything that is decoupled and look at the forces applied, and to what masses.
-Chris
I've gotta say that I'm astounded the discussion is still going......
The plane gets its propulsion from moving the air above the conveyor.
The plane will therefore accelerate at the same speed it would normally, it's just that the wheels are still in contact with the conveyor up to the point where the plane creates enough airspeed to lift itself from the bounding force we call gravity!
I originally though about the wheel speed and the high probability of overheating due to excessive rotational speed…grease ain’t a miracle worker and the higher the revs the higher the heat until……..Cease! This would effectively be like slamming the anchors on and probably would damage the undercarriage enough to cause catastrophe…
Then I thought…… the conveyor matches the speed of the wheels, no more.
This means that the speed of the wheels is exactly the same as it would be in normal conditions, as the conveyor does not increase the speed…it only matches it!
Hell, when you look at it there is absolutely no difference between this and a normal take off procedure!
The plane gets its propulsion from moving the air above the conveyor.
The plane will therefore accelerate at the same speed it would normally, it's just that the wheels are still in contact with the conveyor up to the point where the plane creates enough airspeed to lift itself from the bounding force we call gravity!
I originally though about the wheel speed and the high probability of overheating due to excessive rotational speed…grease ain’t a miracle worker and the higher the revs the higher the heat until……..Cease! This would effectively be like slamming the anchors on and probably would damage the undercarriage enough to cause catastrophe…
Then I thought…… the conveyor matches the speed of the wheels, no more.
This means that the speed of the wheels is exactly the same as it would be in normal conditions, as the conveyor does not increase the speed…it only matches it!
Hell, when you look at it there is absolutely no difference between this and a normal take off procedure!
Okay guys, first of all
ALL OF YOU THAT ARE SPEAKING OF "ROLLING RESISTANCE", PLEASE STOP.
In rolling motion, the wheel, in each instantaneous moment, is stationary against the ground. There is no resistance coming from ANYWHERE. Talking about "ROLLING RESISTANCE" is being entirely ignorant of how wheels work.
Allright, back to the topic.
Okay, everyone answer this question. Forget about the original poster's question:
Imagine a plane on a normal runway, no frigg'n conveyer belts anywhere. However, there is a wind, coming from behind the plane. The wind speed is such that no matter how fast the plane is travelling on the ground the wind will match its speed, albeit moving in the same direction as the plane. Will the plane still take off?
ALL OF YOU THAT ARE SPEAKING OF "ROLLING RESISTANCE", PLEASE STOP.
In rolling motion, the wheel, in each instantaneous moment, is stationary against the ground. There is no resistance coming from ANYWHERE. Talking about "ROLLING RESISTANCE" is being entirely ignorant of how wheels work.
Allright, back to the topic.
Okay, everyone answer this question. Forget about the original poster's question:
Imagine a plane on a normal runway, no frigg'n conveyer belts anywhere. However, there is a wind, coming from behind the plane. The wind speed is such that no matter how fast the plane is travelling on the ground the wind will match its speed, albeit moving in the same direction as the plane. Will the plane still take off?
Bobken said:
I just caught this, and this is so absurd that I must comment:
ARE YOU MAD!? Of course the rocket is not sitting vertical on the ground. The rocket is lying sideways, on wheels, with the propulsion parallel to the ground, just like the plane when it is on the ground. The question is whether the rocket in the sense will have a net forward movement, and in this case no. And since the plane need a net forward movement in relation to the air in order to produce lift.... you get the picture
And one does not become director of a large scientific facility (nevermind that he is also the founder) by being loose in physical analysis. And even if it escaped him, it wouldn't have made it past the TA and the various collaborators responsible for 1st year honours physics material if it isn't true.
Dear eVITAERC,
I'm a bit tired of giving explanations you don't seem to read anyway... Instead you come with new ideas and, sorry for that, senseless statements: Of course there is something like rotational friction (to the others: it's Rollreibung in german, not sure whether I translated it correctly), a deeper look in any physics book covering simple classical mechanics will tell you so...
But I now, all these authors are stupid ignorants who haven't understood the physics of wheels.. Well then, congratulations, you and your lecturer seem to be the only 2 persons in the world who really know about the physics of rolling wheels. Go ahead, there should be plenty do to with this knowledge
...
------------
Short remark to your second question, no, the plane will not take off in that situation, because it has no chance to reach its minimum take-off speed relative to the sourrounding atmosphere... als long as the wind from behind always matches the plane's speed (relative to ground) there is no air flow around the wings and no lift created...
Greetings,
Andreas
I'm a bit tired of giving explanations you don't seem to read anyway... Instead you come with new ideas and, sorry for that, senseless statements: Of course there is something like rotational friction (to the others: it's Rollreibung in german, not sure whether I translated it correctly), a deeper look in any physics book covering simple classical mechanics will tell you so...
But I now, all these authors are stupid ignorants who haven't understood the physics of wheels.. Well then, congratulations, you and your lecturer seem to be the only 2 persons in the world who really know about the physics of rolling wheels. Go ahead, there should be plenty do to with this knowledge
...------------
Short remark to your second question, no, the plane will not take off in that situation, because it has no chance to reach its minimum take-off speed relative to the sourrounding atmosphere... als long as the wind from behind always matches the plane's speed (relative to ground) there is no air flow around the wings and no lift created...
Greetings,
Andreas
Hi eVITAERC,
I am so glad I am not on a jury with you. You did hang this thread.
I will restate my previous comments to you:
Forget everything about your physics question and use your powers of deduction. For the record, I believe all of us passed our post secondary physics course (those who had it). I know I did.
If that's not enough, look at how many intelligent people who have replied holding a different view than yourself. Perhaps you should email this question to your ex-prof?
-Chris
I am so glad I am not on a jury with you. You did hang this thread.
I will restate my previous comments to you:
Forget everything about your physics question and use your powers of deduction. For the record, I believe all of us passed our post secondary physics course (those who had it). I know I did.
If that's not enough, look at how many intelligent people who have replied holding a different view than yourself. Perhaps you should email this question to your ex-prof?
-Chris
Rundmaus said:Dear eVITAERC,
I'm a bit tired of giving explanations you don't seem to read anyway... Instead you come with new ideas and, sorry for that, senseless statements: Of course there is something like rotational friction (to the others: it's Rollreibung in german, not sure whether I translated it correctly), a deeper look in any physics book covering simple classical mechanics will tell you so...
But I now, all these authors are stupid ignorants who haven't understood the physics of wheels.. Well then, congratulations, you and your lecturer seem to be the only 2 persons in the world who really know about the physics of rolling wheels. Go ahead, there should be plenty do to with this knowledge
------------
Short remark to your second question, no, the plane will not take off in that situation, because it has no chance to reach its minimum take-off speed relative to the sourrounding atmosphere... als long as the wind from behind always matches the plane's speed (relative to ground) there is no air flow around the wings and no lift created...
Greetings,
Andreas
Dear Andreas,
"Rollreibung", if I read it right, comes form the realm of engineering stating that a real wheel will always work against friction in the bearings, as well as against minor surface deformations at the wheel and the ground. It is, however, a purely engineering issue in that the wheel bearings can always be designed well enough to have non-negligible friction, and the tire surface can always be made hard enough and unifrom enough such that "Rollreibung" or "Rolling friction" (not rotational friction) is minimized to be negligable.
This in no way address my beef with most people's usage of "rolling friction", which seem to imply that a moving object on wheels will simply lose energy to the wheels due purely to the nature of rolling. This isn't so. A rolling wheel, if designed ideal enough, can do no work due to friction as the tires are constantly "catching" the ground.
Btw, I do not "keep coming up with new ideas". I'm simply reading what everyone's "proof" of why the plane will take off, noticing inconsistencies, and try to describe in more detail the different aspect of this physical system in order to adress the general misconceptions of kinematics that seem to arise. It may seem like I'm jumping from place to place, but really I'm talking about the same thing. I know my wordings can be really confusing and usually I don't drive the main points enough, but bear with me as I obviously don't have much experience writing physics texts.
Regarding your second part, if you think that things will work out that way in the scenario I provided, then I can prove to you that the plane can never take off, according to the laws of classical relativity 🙂 (the classical one ala Gallileo, nothing to do with Einstien)
****
But now it's off to build my second Millet Hybrid with 12AE6A tubes 😀 So any more expounding on this already too tedious topic will have to wait till later~
OK maybe since I have seen the light I can spread a bit of it 😉
I was incorectly thinking that the force the conveyor applied had to be fully counteracted by the thrust from the engines. I was going on the old rules like for every action there is an equal and opposite reaction, and conservation of momentum....
If the wheels cannot rotate then this stands, but because the wheels DO rotate, the majority of the force being applied by the belt is converted into angluar momentum of the wheel, this has no effect on the plane itself. as Chris pointed out it is effectively decoupled from the plane by its bearings.
so momentum is being conservered and the plane will take off 🙂
Tony.
I was incorectly thinking that the force the conveyor applied had to be fully counteracted by the thrust from the engines. I was going on the old rules like for every action there is an equal and opposite reaction, and conservation of momentum....
If the wheels cannot rotate then this stands, but because the wheels DO rotate, the majority of the force being applied by the belt is converted into angluar momentum of the wheel, this has no effect on the plane itself. as Chris pointed out it is effectively decoupled from the plane by its bearings.
so momentum is being conservered and the plane will take off 🙂
Tony.
anatech said:
I did think really hard about this, and what I stated is the answer I reached.
The problem with trying to isolate systems using wheels is that many forces act on a rolling wheel, but only some of them do work and thus only some of them can provide propulsion. Tis makes a wheel confusing to analyse. Another issue with wheels is that it still couples the plane in the sense that the ground as well as the wheel must be analysed together as one system ("rolling free-body doesn't make sense, it'll be called "rotating free body). This means that a rolling wheel is sensative to the relative motions of the ground, which is the main beef in this situation.
And as long as the wheel is coupled to the plane by a landing gear, such that it is free to rotate but not free to go forward on its own, it also must be in the same system with regard to forward motion. No matter what produced the propulsion.
If you still think that the wheel is "decoupled" from the plane, then I can only say that you are right. But only in the sense of rotational motion. In rolling motion where the wheels also go forward as well as round and round, the wheel cannot roll forward unless the plane also move forward, and vice versa.
Passing physics course does not automatically grant you a pass in analysing things. Many people in my class got that question wrong (more than 80%) but still all of them passed.
Hi eVITAERC,
True, and I stated that the energy loss due to the bearings in the wheel was minimal. Since the error term is small and we have excess thrust, we are certian to reach an air speed across the air foil where lift exceeds the force of gravity. Also, as the air velocity increases, the gravitation load on the wheel bearings is reduced.
The flight has not been delayed and will reach the destination as planned.
Now if you are saying the energy loss in the bearings is very significant we would see many accidents due to bearing failure. This would also cause far more tire failure.
-Chris.
True, and I stated that the energy loss due to the bearings in the wheel was minimal. Since the error term is small and we have excess thrust, we are certian to reach an air speed across the air foil where lift exceeds the force of gravity. Also, as the air velocity increases, the gravitation load on the wheel bearings is reduced.
The flight has not been delayed and will reach the destination as planned.
Now if you are saying the energy loss in the bearings is very significant we would see many accidents due to bearing failure. This would also cause far more tire failure.
-Chris.
anatech said:
Allright I'll try to talk in this language.
In your sense, the "friction loss" is INFINITE. In fact it's not right to call it a "energy loss", since the plane is simply pushing against a "wall of sorts". Saying that you can move forward irrgardless of what the wheel is doing is ignorant. Here we are trying to push radially on a circular bearing. You simply won't move at all if the bearing is fixed at some place. The way you can even move at all is if we moved the bearing along with you, which on a plane is accomplished by the wheel translating your radial pushing into a rolling motion.
So the rolling mostion has everything to do with moving forward. It's not as "decoupled" as you might think.
eVITAERC said:
OK the key point here is that the conveyor belt is not powered... perhaps if you realise why this is so completely wrong you will see why your argument is wrong 🙂
The belt is not powered, it is frictionless and massless. the only way to get the belt to move would be to exert some force on it. What is in contact with it?? the wheels, when the rocket fires is there some majic transference of the torque from the rocket engine to the wheels??? NO! If it was a turbine that was driving a gearbox which drove the wheels, then yeah it would stand still, but it isn't, the force is the rocket engine, If the wheels were fastened to the conveyor (since the belt isn't powered) then the rocket would pull the belt forward!!! if they aren't fastened then the wheels will simply roll forward being pulled along by the rocket, if the force is being applied by the wheels rotatiing then that is the wheels pushing, and they would push the belt away, with the whole device remaining stationary.
I think you have had a memory failure, and suggest you consult your ex lecturer, so you don't continue believing something that is completely wrong 🙂
Tony.
wintermute said:
Ah but what is the belt doing.....
Is it moving at the same tangential speed as the wheel, but in the opposite direction!
When you specify that, you use up all the degree of freedom on a rotating object. In every sense, what the conveyer belt is doing in that quesiton and what it is doing here is very much the same thing. The quesiton didn't specify whether the conveyer belt is powered, so it could very well be not at all.
I'll drop a another hint, as much as I hate to artificially inflate my post count (what post count?) in this way:
Try to observe what is happening from the POV of the conveyer belt. Imagine that you are strapped to the conveyer belt. Since the air is still relative to the ground, do you now feel the wind moving at you? What direction is it moving at?
Now look at the plane. It's speeding away from you, just as you expect it to if you and the plane were both on the ground.
Notice the wind? What is it doing as the plane is speeding up?
Now add the postulate of classical relativity: You must always see the same thing happen no matte rwhat spee you're travelling at. Mind you, onjects would be moving at different velocities relative to you, but the same result must happen.
Okay me go build Millet now, for real this time 😉
Try to observe what is happening from the POV of the conveyer belt. Imagine that you are strapped to the conveyer belt. Since the air is still relative to the ground, do you now feel the wind moving at you? What direction is it moving at?
Now look at the plane. It's speeding away from you, just as you expect it to if you and the plane were both on the ground.
Notice the wind? What is it doing as the plane is speeding up?
Now add the postulate of classical relativity: You must always see the same thing happen no matte rwhat spee you're travelling at. Mind you, onjects would be moving at different velocities relative to you, but the same result must happen.
Okay me go build Millet now, for real this time 😉
Did you bother to read the rest of what i said, or my previous post about the majority of the energy of the belt being converted into angular momentum of the wheel.....
I give up, I'm happy that I have made the right decision 🙂
someone is going to be very embarrased over all this I suspect!
Tony.
I give up, I'm happy that I have made the right decision 🙂
someone is going to be very embarrased over all this I suspect!
Tony.
Hi eVITAERC,
Are you telling me that you were able to sneak up and weld the wheel bearings so they would not rotate?
-Chris
Edit: corrected word
Are you telling me that you were able to sneak up and weld the wheel bearings so they would not rotate?
-Chris
Edit: corrected word
Physics 101
Whew!
I've read 10 pages of this and hopefully someone else has thought of this:
Newton's Law of Motion.
The Conservation of Energy.
The Law of Inertia.
A body in motion tends to move forward in a straight line unless acted upon by another force. That's how a jet, rocket or prop engine works.
Forget engines pushing or pulling air across the wings, it's the forward motion of the plane that slices the wings through the air which creates lift. The motion of the landing gear/wheels has absolutely nothing to do with the speed of the wings through the air. It's the speed/motion of the wings through the air (peat and repeat where in a boat) that creates drag and lift!
The engines create a force which MUST be dealt with! The engines MUST tend to move forward and since the plane (wings) is/are attached to the engine, they/it must tend to MOVE forward also, unless you anchor/attach the plane to the ground, which we are not doing here with the conveyor belt! If the plane was anchored to the ground the energy would be transferred to the anchors. If the energy were great enough (without tearing the plane apart!) it would break the anchors and the plane would move forward and take off. Duh! :B
The hot gases coming from the back of a jet engine doesn't push against anything, think of rockets in the vacuum of outer space!
CU
Wayne
Whew!
I've read 10 pages of this and hopefully someone else has thought of this:
Newton's Law of Motion.
The Conservation of Energy.
The Law of Inertia.
A body in motion tends to move forward in a straight line unless acted upon by another force. That's how a jet, rocket or prop engine works.
Forget engines pushing or pulling air across the wings, it's the forward motion of the plane that slices the wings through the air which creates lift. The motion of the landing gear/wheels has absolutely nothing to do with the speed of the wings through the air. It's the speed/motion of the wings through the air (peat and repeat where in a boat) that creates drag and lift!
The engines create a force which MUST be dealt with! The engines MUST tend to move forward and since the plane (wings) is/are attached to the engine, they/it must tend to MOVE forward also, unless you anchor/attach the plane to the ground, which we are not doing here with the conveyor belt! If the plane was anchored to the ground the energy would be transferred to the anchors. If the energy were great enough (without tearing the plane apart!) it would break the anchors and the plane would move forward and take off. Duh! :B
The hot gases coming from the back of a jet engine doesn't push against anything, think of rockets in the vacuum of outer space!
CU
Wayne
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