Cue PH brainiacs - logic puzzle!
Discussion
esselte said:
.... and eventually gain enough speed to produce lift and take off as normal
>> Edited by swilly on Tuesday 13th December 14:03
If the conveyor speed keeps up with the wheels rotation speed how will it gain airspeed?
That statement is irrelevant. The wheels DO NOT move the jet forward, they are not powered, the thrust DOES which is against the air and independent of the ground, conveyor etc. The wheels are simply a practical way of preventing the jet sitting on the ground and dragging against it.
swilly said:
.... and eventually gain enough speed to produce lift and take off as normal
>> Edited by swilly on Tuesday 13th December 14:03
If the conveyor speed keeps up with the wheels rotation speed how will it gain airspeed?
That statement is irrelevant. The wheels DO NOT move the jet forward, they are not powered, the thrust DOES which is against the air and independent of the ground, conveyor etc. The wheels are simply a practical way of preventing the jet sitting on the ground and dragging against it.
well, honestly. why didnt somebody just say that before?
And the correct answer is....maybe it could take off if the jets are powerful enough and positioned behind the wings.
Basically the suction produced by the jets would cause airflow over the wings. Enough of that (some stupendously powerful engines required) would be sufficient for a vertical take off (actually vertical only for an instant, once the wheels have left the conveyor the plane would rocket forward).
Basically the suction produced by the jets would cause airflow over the wings. Enough of that (some stupendously powerful engines required) would be sufficient for a vertical take off (actually vertical only for an instant, once the wheels have left the conveyor the plane would rocket forward).
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
What happens once the plane is moving, can we vary the speed of conveyor perhaps going backwards and forwards without affecting the ground speed? Will it take off any quicker if the conveyor belt was going in the same direction as the plane? How do aircraft carrier slings work?
As I understood, the question was "what happens if the conveyor belt matches the ground speed." However technically difficult it maybe to match the speed of the conveyor belt that is what the question asked.
And if the conveyor is matching the ground speed of the plane, it will not take off (I think!) and I'm not listening to anyone that says otherwise!
What happens once the plane is moving, can we vary the speed of conveyor perhaps going backwards and forwards without affecting the ground speed? Will it take off any quicker if the conveyor belt was going in the same direction as the plane? How do aircraft carrier slings work?
As I understood, the question was "what happens if the conveyor belt matches the ground speed." However technically difficult it maybe to match the speed of the conveyor belt that is what the question asked.
And if the conveyor is matching the ground speed of the plane, it will not take off (I think!) and I'm not listening to anyone that says otherwise!
boiler said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
What happens once the plane is moving, can we vary the speed of conveyor perhaps going backwards and forwards without affecting the ground speed? Will it take off any quicker if the conveyor belt was going in the same direction as the plane? How do aircraft carrier slings work?
As I understood, the question was "what happens if the conveyor belt matches the ground speed." However technically difficult it maybe to match the speed of the conveyor belt that is what the question asked.
And if the conveyor is matching the ground speed of the plane, it will not take off (I think!) and I'm not listening to anyone that says otherwise!
please. youre all really hurting my head now.
boiler said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
Er, that would be the throttle then. And the brakes. Same as a car.
>> Edited by Gasblaster on Tuesday 13th December 15:02
andy mac said:
Why? The speed the conveyer belt is going bears no relation to the force the engines are exerting upon the air. The aircraft exerts it's force on the air around it, not on the ground. the aircraft would move forward, despite anything the conveyer belt was doing.
Give it up mate. There is no hope. Global warming will kill them all (with any luck).
>> Edited by xm5er on Tuesday 13th December 15:07
swilly said:
The answer to the conveyor/jet question is that the plane takes off.
The planes wheels are simply there to support the plane and allow it to move on the ground. The wheels arent powered.
A car for example would stay still because the conveyor moving backwards would negate the traction and friction of the powered wheels against the ground in the opposite direction, just like a rolling road.
The jet on the other hand relies on its thrust against the air to push it forward. The jet moves backwards whilst it produces no thrust. Once it starts producing thrust it would need nominal time to counter and negate the backwards force present at that time BUT the action of the conveyor becomes irrelevant once the jet is thrusting. As the conveyor speeds up the jet wheels just turn faster BUT the jets motion would begin to move forwards as its thrust is against the air and eventually gain enough speed to produce lift and take off as normal
>> Edited by swilly on Tuesday 13th December 14:03
are you taking the p*ss?
boiler said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
What happens once the plane is moving, can we vary the speed of conveyor perhaps going backwards and forwards without affecting the ground speed? Will it take off any quicker if the conveyor belt was going in the same direction as the plane? How do aircraft carrier slings work?
As I understood, the question was "what happens if the conveyor belt matches the ground speed." However technically difficult it maybe to match the speed of the conveyor belt that is what the question asked.
And if the conveyor is matching the ground speed of the plane, it will not take off (I think!) and I'm not listening to anyone that says otherwise!
I refer the honourable gentlemen to my answer above.
The only thing that matters is if there is sufficient airflow over the wings to produce lift in excess of the aircraft weight.
Normally that is acheived by accelerating down a runway until the airspeed is sufficient (groundspeed irrelevant).
It may be possible to produce sufficient airflow with adequately powerful and positioned jet engines drawing in air from the front.
Gasblaster said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
Er, that would be the throttle then. And the brakes. Same as a car.
>> Edited by Gasblaster on Tuesday 13th December 15:02
And if you go to somewhere like London City airport, because of limited runway space, they put the brakes on, throttle up to (around 85% i believe) and then let the brakes off, so that they do not waste the space. This is exactly what would happen in a car, but then we have accepted that the car would remain stationary.
boiler said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
Er, that would be the throttle then. And the brakes. Same as a car.
>> Edited by Gasblaster on Tuesday 13th December 15:02
And if you go to somewhere like London City airport, because of limited runway space, they put the brakes on, throttle up to (around 85% i believe) and then let the brakes off, so that they do not waste the space. This is exactly what would happen in a car, but then we have accepted that the car would remain stationary.
A car remains stationary because its forward motive force is applied to the ground, which in this case is moving backwards and cancelling it out.
A plane takes off because its forward motive force is applied to the air, which is not affected by the ground (conveyor) moving backwards. See my ice example above.
GreenV8S said:
You have a 1/3 probability of picking the right door at your first guess. Therefore a 2/3 probability that one of the other two doors is the right one (at this stage you can't tell which; each door has an equal 1/3 probability of being the right one).
It isn't explicitly stated, but I think it's implicit that the host will choose to open a wrong door. After they've done this, there is still a 1/3 chance that the door you originally picked is right, but now a 2/3 chance that the other door is right.
The reason it's 2/3 not 1/2 is that the host added some information by looking at the two doors you didn't pick, and telling you that one of them was the wrong one. Therefore the probability that the remaining one is the right one is the same as the probability that one of them is the right one.
No, that's because you are considering the odds from two different points in time.
The moment the losing door is opened, the odds that the door you had chosen hides the car change from 1/3 to 1/2, because one door has been removed, but there are the same number of winning doors.
The moment the losing door is opened, the odds that the door you had chosen does not hide the car change from 2/3 to 1/2, because one door has been removed, and one of the losing doors has been removed.
Alternatively, think of every possible scenario (e.g. Choose 1, Host opens 2, you stick, Car is behind 3), and work all of them out. Then look at all the possible outcomes (You have Door1, Car is behind Door 3). You will find that several scenarios (particularly when your first choice Door is in fact the winning door, and therefore there are odds on which losing door the host will open) produce the same outcome. You will then find that the number of outcomes where you win by sticking is in fact exactly the same as the number of outcomes you win by switching. If you follow this route, what happens in between is irrelevant.
The sleight of hand in this problem is that you do not consider all the possible scenarios and the probability of each of those scenarios. The 'solution' does this by saying "For the moment, we will make the simplifying assumption that The Contestant always begins by choosing Door #1 - As we will see, this assumption does not affect the answer" - Not True. Not least because he has ignored a scenario that is relevant.
The web is full of sites about the Monty Hall problem, every last one that I have visited has supported the analysis made by GreenV8S and myself - if you can find a single site supporting your analysis HiRich I shall consider it some more, but atm I'm convinced that GreenV8S and I are correct, and I think you'll have some difficulty finding that site.
Gasblaster said:
So if these jets are producing sooooo much thrust that the plane will take off anyway, how does the friction between the tires and the ground hold this immense thrust prior to take off?
Er, that would be the throttle then. And the brakes. Same as a car.
>> Edited by Gasblaster on Tuesday 13th December 15:02
And if you go to somewhere like London City airport, because of limited runway space, they put the brakes on, throttle up to (around 85% i believe) and then let the brakes off, so that they do not waste the space. This is exactly what would happen in a car, but then we have accepted that the car would remain stationary.
A car remains stationary because its forward motive force is applied to the ground, which in this case is moving backwards and cancelling it out.
A plane takes off because its forward motive force is applied to the air, which is not affected by the ground (conveyor) moving backwards. See my ice example above.
ARGHHHH...
The forward force is provided by thrust, yes.
The drag force is the friction between the wheels and the ground, and friction in the wheel bearings.
So even if we have a 747 load of thrust, if we have the conveyor travelling at a billion mph, then the friction in the wheel bearings and rolling friction in the tyre/conveyor interface is enough to equalise the force of thrust.
How hard is it to understand?
The question states the conveyor goes as fast as possible to match the 747's speed, ie, the 747 is going 0mph relative to the world, and so is the conveyor. To keep the relative speeds 0, it will go as fast as possible.
Rolling friction is what makes a car top out along with aero drag. Ever fitted wider tyres to a car and noticed it accelerates slower at speed?
The aircraft will not go anywhere if the conveyor can generate enough rolling friction through the wheels to offset the thurst.
We all know in real REAL life it would take off, as no such conveyor exists, but neither does a closed system where the conveyor wouldn't literally *drag* the air over the wings generating "air speed"... allowing the aircraft to hover relative to the world within the shaft of moving air over the conveyor!
Simple A level physics. To not move you oppose force with something. The wheels do this on a car, on a taxi'ing or taking off plane, along with lots of other small forces, so they can all be used to offset the thrust. In this case rolling drag is proportional to rolling velocity.
Just spin the wheels really quickly and your thrust is countered and the aircraft sits there.
Dave
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