Physics Puzzle

[ban-all-sheds]

the treadmill is a good example bas but you are missing the point again in what you have just pointed out, ie you mention the person running acting against the treadmill, this is the energy expended in order to keep stationary(the thrust) as you said but there is the answer, the person is stationary in airspace bar the up and down motion associated with running.

No - it's a terrible example in the context of this question. Walking is a complicated business, involving gravitational and frictional forces, but ultimately you move in relation to the belt because of forces you apply to the belt.

With the plane, it does not move because of any forces acting on the ground.

the winch analogy doesn't help the problem as that it is a forward acting force not a rear acting force propelling the plane

A WINCH AND AN ENGINE PRODUCE FORCES ACTING ON THE PLANE IN THE SAME DIRECTION!!!!!!!!!!!!!!!!!! FORWARDS!!!!!!!!!!!!!

and doesn't allow the plane any backward motion, look at it this way the thrust is like a hand holding the plane from moving backwards do you agree the plane woul;d go backwards if their was no thrust but the runway control gear was set to moving, the wheels of the plane would be static and the whole craft would move backwards.

Yes, I agree.

the thrust is what is trying to counteract this backwards force and because of it the wheels turn. now because the wheels are turning they are gaining ground ie the circumference of the wheel is being applied to the ground each revolution, if the ground was static the plane would move forward , the trouble is for every revolution gained the ground has shifted backwards the same, hence the plane in effect has gained no ground it is in exactly the same place it was when it started. no air going over the wings means no lift.

No - that's false.

It would be true if the planes forward motion relative to the air derived from forces transmitted through the wheels, but it isn't.

I ask you again - in your model, what happens when a plane leaves the runway? If its forward motion depends on forces acting on the wheels, why does it carry on moving forward?

The answer is that it carries on moving forward because the forces acting on the plane have nothing to do with the wheels. So if they have nothing to do with the wheels after takeoff, they have nothing to do with them before takeoff, and it is irrelevant how the wheels move relative to the runway, the plane will still move forwards through the air.

Think of surveyors walking around measuring distance, they do it by pushing a wheel on a stick around, it's a physical thing, the mechanics of the wheel are a measuring device in effect.

OK - think of this - what distance would be measured if the surveyor was on an airport travelator, and walked along it pushing the device?

the cart example i'm not sure i agree with what you say about not being able to push it anymore from what you are saying i would be on the conveyor too?

No - that's the whole point - you are not on the conveyor belt, you are standing alongside it, feet on the ground.

if i was beside the conveyor standing on static ground then i could hold the cart stationary

Yes you could.

But you are claiming that you could no longer walk forwards causing the cart to move forwards relative to the floor.

but it's wheels would spin ever faster and i would expend more energy in trying to do so until eventually i could not balance the equation anymore same as the thrust on the plane there would be a time when the balance could not be maintained either the conveyor would give up or you would reach maximum thrust. but we are talking ideals here and assuming that both the thrust and the conveyor have no limitations ot that they both can attain the balance ie when the plane is at max thrust the conveyor still manages to balance the equation.

No - it cannot - it is impossible.

If that is confusing then forget all about the thrust and forces and just think mechanics, the wheel is a measuring device but in the example what is it measuring? distance travelled of course, on solid ground it would measure the distance it has travelled over this ground and as the solid ground is static it can be used as a reference, so you can tell by the amount of revolutions what distance they have travelled over the ground.
The conveyor is moving the starting reference has shifted you can only say the wheels have travelled over distance in relation to the belt but the physical effect only relates to the belt and wheels.
you as an observer standing on static ground would perceive the ground moving under the plane also the wheels turning but the plane itself would be static to you.

OK - what if the conveyor belt moves forwards, so that the wheels don't turn at all? Would you claim that the plane cannot take off because it is not moving?

maybe the confusion is coming from peoples perception that the wheels are somehow powered? they are freewheeling but the closed loop sensing of the system is looking at what speed these wheels are turning at and adjusting the conveyor accordingly. The only part that we can get a speed refence from regarding how fast the plane is wanting to move is the wheels, ie how fast they are spinning

I don't know where the confusion comes from, - maybe you can tell us, as you are the one who is confused. The only thing I'm confused about is how to explain it to you.........

 

[johnny_t]

Let me start by confessing that I've not read the last three pages since the arguments started, but I've just had a new idea. I'll also confess that it seems a trifle counter-intuitive and probably wrong, but I can't think of a hole in the argument, so its over to you guys.......

Much of the disagreement here comes from being able to conceive what is happening to the wheels.

The original question said that the conveyor is set to counteract the speed of the wheels. When the thrusters fire, the wheels, as a side result try to roll forwards as well. To counteract this, you have to roll the wheel backwards. To roll the wheel backwards, you have to move the conveyor belt FORWARDS.

So, I propose that firing up the thrusters not only causes the plane to move forwards for reasons previously discussed, but the whole conveyor moves forwards whilst the wheels stay (somewhat precariously) stationary on top of it.

I know it sounds daft, but I can't find the hole.......

 

[Softus]

Much of the disagreement here comes from being able to conceive what is happening to the wheels.

The aircraft moves in one direction, while the conveyor belt moves in the opposite direction.

Notwithstanding that, there are those who believe that what happens at the wheels is significant, and those who believe that it isn't.

Whatever the direction of travel of the belt, it isn't significant.

 

[johnny_t]

Much of the disagreement here comes from being able to conceive what is happening to the wheels.

The aircraft moves in one direction, while the conveyor belt moves in the opposite direction.

Notwithstanding that, there are those who believe that what happens at the wheels is significant, and those who believe that it isn't.

Whatever the direction of travel of the belt, it isn't significant.

Its a fair cop. To be honest, I'd had the idea for about 30 hours and hadn't been able to get a sniff of the internet in that time and was bursting to put it down in writing, so maybe lacked some of the due diligence. Now I've read through more thoroughly, BAS has also touched on this idea as well.

I've had one last thought regarding how to explain the insignificance of the wheels, and it involves BAS's trolley on a conveyor belt. If you put a hand behind the trolley, then you obviously stop the trolley going backwards. Crank up the conveyor and the trolley does not get any harder to hold back - Why ? Because the motion of the conveyor does not resolve itself into a backwards force on the trolley in these circumstance, merely a rotational force on the wheels..


(little edit to take into account Softus' next post - )

 

[Softus]

Full respect for your magnanimous reply

To be pedantic (which comes all too easily to me), what I said was that what happens at the wheels is not significant. I meant by this that there is a non-zero force that is negligent in the context of the opposing jet force.

In the trolley-and-hand analogy, the force on the wheels has two components:

1. The frictional 'resistance' of the bearings is such that some of the force from the belt will be conveyed to the trolley.

2. Once the the wheels are turning, if the tyres are rubber, then some force is required to compress that rubber where it meets the belt.

In the context of the force from a hand that's restraining the trolley, these are small forces. When translated to the airplane scenario, although the energy losses through rubber compression and bearing friction are bigger, the jet engine is hugely more powerful than a hand, which is why I claim that what happens at the wheels is not significant. They aren't non-existent, they're just not significant.

 

[Pensdown]

Another example.

The plane is flying at 100ft above the conveyer belt which is running backwards at full speed. Obviously the conveyer has no effect on the planes movement.

The plane is now flying normally 5ft above the conveyer which is still running backwards at full speed. The pilot continues to fly but just for fun he drops the wheels so they come into contact with the conveyer belt. Nothing would change, the plane would carry on flying and the wheels would spin freely as they hit the conveyer.

Take off is the same principle

 

[2scoops0406]

Err, no. I think you are all reading too much implied "knowledge" into the question. From a scientific point of view, answer the question with the data supplied, do not assume anything.

Because the wheels of the plane move, this assumes that there is friction. As they move, the co-efficient of friction must be greater than zero. As you have no other information, you have to assume that the co-efficient of friction is one. therefore any thrust generated by the engines, directly or indirectly applied to the wheels must result in an equal an opposite force being applied to the amount of thrust being produced. Net result, no movement, simple convervation of momentum.

Bearing friction, etc, not given, therefore assume is zero, the plane will not move, not a real world scenario, but otherwise you're acting as engineers, not scientists, you're not answering the question.

 

[markie]

Is this fooking topic still going,

 

[Nige F]

Yeah But the Ferkin plane`s outa fuel

 

[kendor]

bas in reply to your post you mention the wheels have no part to play but they do, the tachometer or whatever sensing device being used to control the runway is looking at them.
you mentioned the conveyor and my suggesting i couldn't walk forwards i'm not suggesting anything like that, let me explain again, i could walk forwards if i wanted to and gain ground i understand you saying that my intervention is pushing the vehicle forward and that this could be interpreted as the thrust but i believe the analogy is flawed because your influence is upsetting the balance, with the plane example we are lead to believe that the runway always balances or cancels out any increase in thrust. the only reference i can see in the example is the distance the wheels travel which although spinning are not gaining any ground and the plane doesn't move in airspace.

although the wheels are not powered and are freewheeling there has to be traction to the ground(moving runway) otherwise the wheels wouldn't turn but merely skid (what would happen if the hand brake or whatever it is called in avionics was applied?)and this traction allows a measurement ie say the wheels turn 500 revolutions then the plane will have moved ahead 500 circumferences of the wheel, but the runway has moved 500 circumferences backwards and cancelled out the travel of the wheels in respect to the static surroundings (buildings etc) if you say the wheels are replaced by skids or something like it that have no traction then i will agree the plane will move forward. but for the example we have to assume that the runway and the wheels are of a similar material that has the same frictional factor such as rubber for example.

i like your example of the surveyor on the travelator, you asked me what distance had he travelled here's thr tricky bit, in relation to what?
indeed he has travelled a distance as observed by the wheel and the wear of his soles but how far has he travelled up the airport corridor, i bet he's no nearer the lounge than when he started( we have to assume the balancing effects in the plane problem are in force here ie the travelator perfectly matches the speed he's walking at).

In your examples of the conveyor etc what reference of distance travelled are you using? how do you measure the planes travel in airspace by what reference point?

 

[Softus]

An aircraft is standing on a very long runway that can move (a conveyor belt). The aircraft moves in one direction, while the conveyor belt moves in the opposite direction. This conveyor belt has a control system that tracks the aircraft's wheels speed and tunes the speed of the conveyor belt to be exactly the same as the wheels, but in the opposite direction.

...with the plane example we are lead to believe that the runway always balances or cancels out any increase in thrust.

This is the nub of your misunderstanding.

The conveyor runs in the opposite direction to the travel of the plane. Nothing in the original post said that the force created by the conveyor is an opposite and equal reaction to the thrust.

how do you measure the planes travel in airspace by what reference point?

The original question asked whether or not the plane would ever take off, so measure that by any point of reference you like that enables you to answer the question.

 

[big-all]

ok a lateral thinking here dont know if this analergy has been tried

lets replace the convayor belt with a skate board with extra large smooth wheels and zero air resistance in a forward motion
which is identical to a conveyor trying to"drag backwards"

now there will be a very gradual slow down but only very very slowly as as theres close to nill friction in the bearings
you can only trasfer traction at the rate friction permits and roller bearings have very little friction coupled to the rolling resistance of the tyres [perhaps 5 or 10% of forward effort]so wouldnt stop a skateboard quickly

and cerainly wouldnt stop a jet because the even if it was going backwards at 200 miles an hour as soon as you fire up the jet engines the air resistance and jets would soon equilise and the plane would soon become stationary and quickly excelerate forward to takeoff speed

 

[ban-all-sheds]

bas in reply to your post you mention the wheels have no part to play but they do, the tachometer or whatever sensing device being used to control the runway is looking at them.

Yes, but it doesn't matter whether the motion of the runway causes the wheels to revolve in the same direction as normal, only faster, or slower, or not at all, or in the opposite direction to normal - the plane is still going to move forwards because the wheels have no part to play in applying a force to the plane - it's the engines that do that.

you mentioned the conveyor and my suggesting i couldn't walk forwards i'm not suggesting anything like that,

Yes you are. You moving forwards is just like the plane moving forwards, so if you can move forwards so can the plane. You are saying that the plane cannot move forwards, and therefore you are saying that you cannot.

let me explain again, i could walk forwards if i wanted to and gain ground

Then so can the plane.

i understand you saying that my intervention is pushing the vehicle forward and that this could be interpreted as the thrust

It could indeed, as that's just what it is - you are thrusting the cart forwards, therefore it moves forwards with respect to the ground, and it doesn't matter what the wheels are doing on the conveyor belt.

but i believe the analogy is flawed because your influence is upsetting the balance,

No more than the plane's engines.

with the plane example we are lead to believe that the runway always balances or cancels out any increase in thrust.

No we are not.

the only reference i can see in the example is the distance the wheels travel which although spinning are not gaining any ground and the plane doesn't move in airspace.

The plane does not "gain any ground" with respect to the runway.

It does gain ground with respect to everything else, therefore it moves through the air, and therefore it takes off.

although the wheels are not powered and are freewheeling there has to be traction to the ground(moving runway) otherwise the wheels wouldn't turn but merely skid

They would, but assuming that that didn't result in lateral forces acting on the plane, why would skidding wheels prevent the plane from taking off?

(what would happen if the hand brake or whatever it is called in avionics was applied?)

That might create frictional forces large enough to prevent the plane from moving, or moving fast enough to take off.

and this traction allows a measurement ie say the wheels turn 500 revolutions then the plane will have moved ahead 500 circumferences of the wheel, but the runway has moved 500 circumferences backwards and cancelled out the travel of the wheels in respect to the static surroundings (buildings etc)

In which direction do you think the runway is moving?

if you say the wheels are replaced by skids or something like it that have no traction then i will agree the plane will move forward.

But with aircraft wheels engineered to have very little rolling resistance, i.e. almost frictionless, you don't think the plane will move at all???

but for the example we have to assume that the runway and the wheels are of a similar material that has the same frictional factor such as rubber for example.

i like your example of the surveyor on the travelator, you asked me what distance had he travelled here's thr tricky bit, in relation to what?
indeed he has travelled a distance as observed by the wheel and the wear of his soles but how far has he travelled up the airport corridor, i bet he's no nearer the lounge than when he started( we have to assume the balancing effects in the plane problem are in force here ie the travelator perfectly matches the speed he's walking at).

Hmmm. I didn't say in which direction he was walking...

Try this one:

A man is rowing a boat upstream, along a stretch of river where it's speed is constant, and he passes under a bridge. 1 mile further upstream, he passes under a second bridge, and as he does so his closed tupperware sandwich box falls into the river.

He doesn't notice this immediately - in fact he carries on rowing for 5 minutes before he does. When he realises, he turns round, and begins rowing after his sandwiches, and he catches up with them as they pass under the first bridge. Assuming that he has not rowed any harder on the return trip, how fast is the river flowing?

In your examples of the conveyor etc what reference of distance travelled are you using? how do you measure the planes travel in airspace by what reference point?

That of the air - that's what airspeed is.

 

[Stoday]

You've all missed the significant factor.

The original proposition was that the conveyor moves at the same speed as the wheels, but in the opposite direction.

The periphery of the wheel moves at twice the speed of the wheel as a whole. For example if your car travells at 50 mph, the bottom of the wheel, in contact with the ground, is stationary but the top moves at 100 mph. (all velocities with respect to the ground).

If the ground moves backwards at 50 mph and the wheels move forward at 50 mph, then the wheel spins at twice the speed that it would if the ground were stationary. Thus the top of the wheel travells at (200-50) 150 mph and the bottom at minus 50 mph.

So the plane will take off, provided the wheels can spin at twice their normal take-off speed.

 

[ban-all-sheds]

You've all missed the significant factor.

The original proposition was that the conveyor moves at the same speed as the wheels, but in the opposite direction.

In which direction do you think the runway is moving?