Cue PH brainiacs - logic puzzle!
Discussion
Tinman you are correct about the air passing over the wings creating the lift, and without this the plane will not fly.
However the wheels do not drive the aircraft, the engines do. So what you have is the forward thrust being provided by the engines which are independant of the conveyor. Imagine the same scenario but with a big hand pushing the plane forward - this hand represents the thrust of the engines. Now you can see that all that will happen is the wheels will be travelling at twice their normal speed when the plane takes off, but becuause the hand (thrust) pushing the plane forward is independant of the conveyor the plane can still accelerate forward and create the lift it needs to take off
However the wheels do not drive the aircraft, the engines do. So what you have is the forward thrust being provided by the engines which are independant of the conveyor. Imagine the same scenario but with a big hand pushing the plane forward - this hand represents the thrust of the engines. Now you can see that all that will happen is the wheels will be travelling at twice their normal speed when the plane takes off, but becuause the hand (thrust) pushing the plane forward is independant of the conveyor the plane can still accelerate forward and create the lift it needs to take off
Antwerpman said:
Imagine the same scenario but with a big hand pushing the plane forward
While you're at it, why not imagine a scenario where there isn't a conveyor belt and, instead of an aircraft you have a large vat of porridge connected to the ceiling by bungee cords.
If a big hand comes along and pushes the aircraft forward such that the aircraft achieves true groundspeed and hence airflow and hence lift then you have solved a different problem!
It's like saying "what's 2+2?" "It's 6, so long as the question is 'what's 2+4'".
:smugbastard:Antwerpman said:
Tinman you are correct about the air passing over the wings creating the lift, and without this the plane will not fly.
However the wheels do not drive the aircraft, the engines do. So what you have is the forward thrust being provided by the engines which are independant of the conveyor. Imagine the same scenario but with a big hand pushing the plane forward - this hand represents the thrust of the engines. Now you can see that all that will happen is the wheels will be travelling at twice their normal speed when the plane takes off, but becuause the hand (thrust) pushing the plane forward is independant of the conveyor the plane can still accelerate forward and create the lift it needs to take off
i've never mentioned the wheels in my posts?
wheels have nothing to do with lift.
JonRB said:
Imagine the same scenario but with a big hand pushing the plane forward
While you're at it, why not imagine a scenario where there isn't a conveyor belt and, instead of an aircraft you have a large vat of porridge connected to the ceiling by bungee cords.
If a big hand comes along and pushes the aircraft forward such that the aircraft achieves true groundspeed and hence airflow and hence lift then you have solved a different problem!
It's like saying "what's 2+2?" "It's 6, so long as the question is 'what's 2+4'".
I was trying to make the situation more clear, but thanks for the valuable input I am sure everyone appreciates it.
People seem to forget that the thrust from the engines is independent of the wheels and the conveyor (hence the hand analogy) and that the conveyor has no affect on the stationary air above it. For all real purposes you may as well consider the plane flying over the conveyor because the wheels can transmit so little drag on the plane that this is effectively what it will be doing.
Imagine a person running at 10 mph on a treadmill of the same design as the conveyor, whatever they do they will not move forward because the conveyor matches their leg speed. Now imagine they are again running on the conveyor at 10 mph and their mate (JonRB probably) comes up behind them and gives them a ruddy great shove in the back, also at a speed of 10mph, now they will fly off the front of the treadmill. Why the difference? Because in the first instance their forward motion was dependent upon the force for forward motion being applied through the surface of the treadmill, and in relation to this they were doing 10 mph. In the second instance the forward thrust is applied by a body INDEPENDENT of the surface of the treadmill, so they will move forward at a speed of 10 mph relative to the body applying the force.
For the aircraft it is the same, it applies its forward thrust (force) to the air which is independent of the conveyor surface, and hence the aircraft will move forward relative to the body that it is appling the force to. NO forward force/thrust is applied through the wheels so they are irrelevant
If you want to do some more maths or talk about porridge please feel free
tinman0 said:
Tinman you are correct about the air passing over the wings creating the lift, and without this the plane will not fly.
However the wheels do not drive the aircraft, the engines do. So what you have is the forward thrust being provided by the engines which are independant of the conveyor. Imagine the same scenario but with a big hand pushing the plane forward - this hand represents the thrust of the engines. Now you can see that all that will happen is the wheels will be travelling at twice their normal speed when the plane takes off, but becuause the hand (thrust) pushing the plane forward is independant of the conveyor the plane can still accelerate forward and create the lift it needs to take off
i've never mentioned the wheels in my posts?
wheels have nothing to do with lift.
but the only way the plane can remain stationary and hence have no airflow over the wings (which you did say) is if the wheels are providing the motive power
gasblaster said:
What should you do? And why?
Answer: knock hard on one of the unopened doors.
If you hear nothing, there's probably a car behind there
If you hear a goat, there's probably a goat behind there.
If someone answers, there's probably a stupid stagehand behind there. So ask him "Are you having trouble with that goat?"
jap-car said:
I can't believe I've just read this entire thread
Of course the plane takes off
correct (although the question as stated posits an impossible situation and so has to be moved into the real world to get an answer at all)
jap-car also said:
and of course the chance is 50:50 on the car:goat.
incorrect (providing the gameshow host commits to only ever revealing a goat).
>> Edited by Einion Yrth on Friday 9th December 16:13
Antwerpman said:
Tinman you are correct about the air passing over the wings creating the lift, and without this the plane will not fly.
However the wheels do not drive the aircraft, the engines do. So what you have is the forward thrust being provided by the engines which are independant of the conveyor. Imagine the same scenario but with a big hand pushing the plane forward - this hand represents the thrust of the engines. Now you can see that all that will happen is the wheels will be travelling at twice their normal speed when the plane takes off, but becuause the hand (thrust) pushing the plane forward is independant of the conveyor the plane can still accelerate forward and create the lift it needs to take off
i've never mentioned the wheels in my posts?
wheels have nothing to do with lift.
but the only way the plane can remain stationary and hence have no airflow over the wings (which you did say) is if the wheels are providing the motive power
Um.
If I strap a fan to my back, and stand on rollerskates on a running machine, and set the fan going, I begin to move forward.
The running road now begins to move backwards until I stop moving forwards.
The key here is the friction in the wheel bearings and wheel to road interface. The travelator may well have to move backwards at 10x the airspeed thrust been generated by the fan to generate the frictional force to equalise the speed to zero.
Remember, to accelerate we need a force differential between drag and thrust. If we equal the thrust with drag then we can't accelerate and stay at a constant speed. When the engines reach maximum thrust, then the question states the travelator will always maximise it's speed to cancel out any forward motion.
Using this reasoning, we can assume the wheels of the aircraft can spin faster and faster infinitely until there is adequate rolling resistance to equal the thrust of the engines.
The aircraft will stay still. It can't accelerate, and so it's speed will never move above 0 relative to either the air or ground surrounding the travelator, so it will never take off.
If the wheels are frictionless with the ground then indeed the arguement can be ignored and the aircraft will move and take off.
Sillier still, the travelator spinning at high speeds to offset the thrust of the craft through friction in the wheels would be so high in velocity, that it would literally drag air along and through the wings, providing static lift.
Think of an aircraft taking off with a tail wind, or a headwind. An aircraft carrier moving at 50 knots say into a headwind of 100 knots would in theory launch an aircraft vertically over it's deck... assuming that it's motion relative to the carrier initially was a sum of forces not made up of the wheel's brakes, in which case that force has to be replaced with more engine thrust against the wind.
Very simple. Can't believe so many people argue points even here and start ignoring wheel friction on the aircraft. They are important, they are generating drag proportional to their rotational velocity, countering the aircraft thrust!
Dave
ZR1cliff said:
Come on its been five minutes,,,whats the answer
You select a door - you have 1/3 probability of being correct. call this set A.
There is a 2/3 probability that the car is behind one of the doors, call this set B
the host opens one of the two doors that you did not select, to reveal a goat. Set B now consists of one closed door, and one goat, but there is still a 2/3 probability that set B contains the car.
Therefore if you change your choice from set A (1/3 probability) to set B (2/3 probability) you double your chances of winning.
gasblaster said:
You are on a TV game show. The main prize is a car. You have to choose between three doors. Behind one of the doors is the car. Behind the other two doors are goats. You are asked to choose your door. You choose your door, but the host does not open it. The host then opens one of the other doors (that you did not pick) to reveal a goat. The host then gives you one final chance to change your mind about which door you want. Your choice is to stick with your original choice, or to pick the other unopened door. What should you do? And why?
Ive probably got this wrong (as i often do).. but this looks like a straight 50/50 ?
Starting odds : 1 (choice) / 3 (doors)
Host removes a redundant door : 1 (choice) / 2 (doors)
I would not care if i changed my door or kept it as i have don't know anything about the remaining 2 doors.
Im guessing this is supposed to be a tricky question, so ive probably read the question wrong.
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