People are getting confused by thinking the wheels are spinning at the same speed as the runway. They are picturing a car. Since the wheels are confusing people, picture a plane on ski's.
Please read it and explain to me why the plane won’t fly
__________________________________________________ ________
Forget the wheels
If you built a plane on skis, you could get the plane to accelerate on a flat surface. If the engine is powerful enough you could get the plane to go 140 mph on that flat surface. So, the plane can move 140 mph relative to the ground it is sliding across.
Now if you took that plane and put it on the spinning runway, you could still get the plane to go 140 mph relative to the runway.
Now if the plane is traveling at 70 mph, how fast would the runway have to spin to keep spinning at the same speed the plane is moving?
Since the runway spins at the same speed the plane is moving, the runway would spin at 70 mph.
We have already established that the engines on the plane are strong enough to propel the plane at 140 mph relative to the runway it is on. So the plane will be able to maintain its current speed.
what is the relative speed of the plane to someone on the side lines? 70 mph
how fast would the runway have to spin to keep up with the plane? The runway would be spinning at 70 mph.
What is the relative speed of the plane to the runway? 140 mph. (70mph airspeed + 70mph runway speed in the opposite direction)
Now if this plane can take off at 70 mph airspeed, it would do so.
Quote:
|
Originally Posted by Atooraya
the airplane has to reach a speed where the wheels no longer become a factor for it to take off
i dont see how this is wrong. with the point that an airplane can take off on ski's, fish, dinosaurs, that doesn't matter becuase they'd have to be moving in the opposite direction for that to hold a purpose to this.
The wheels of an airplane are a factor to a certain extent because they allow the airplane to roll forward from the thrust that's created
|
The skis are stationary. They don't move whether or not the runway is in motion or not.
The thrust of the engine can over come the friction caused between the skis and the ground. Just like if you took a snow skier and had him hold on to a rocket. The thrust of the rocket would overcome the drag between the skis and the snow.
If the engine is powerful enough to push the plane when it is on skis when the runway is not in motion, it would be powerful to push the plane half as fast when the runway is in motion. The runway would be in spinning at the speed the plane is moving. So if the plane is moving at 70mph airspeed, and the runway is spinning at 70 mph the other way, the drag caused by the skis touching the ground would be identical to the drag caused by a plane moving at 140mph on a stationary surface.
Quote:
|
Originally Posted by Atooraya
I UNDERSTAND THAT
this is completely irrelevant though!
The airplane is trying to be propelled forward though, but the wheels that are attatched to the aircraft are being spun in the opposite direction, which are creating another force
am i not picturing this properly? i feel like i'm taking CRAZY PILLS
by your logic, and aircraft can be bolted into the ground, and then afterburners be turned on, and the aircraft would still take off because the aircraft isn't propelled forward by anything that is related to the ground or friction or anything
|
Tell me what you understood of my post. I think you misunderstood my post entirely....
Quote:
|
Originally Posted by Atooraya
i understand how airplanes take off, i realize all of it
but all i envision the plane doing in my head is not moving forward because the conveyer belt isn't allowing the airplane to move forward...that's it
maybe i'm picturing it wrong
|
Okay, I'm not disputing the fact you know how a plane takes off.
What I am saying is that the speed the treadmill spins is the same as the speed the plane is moving.
(just humour me and picture this plane sitting on skis, no wheels)
If the plane is moving at 70 mph, then the runway would be spinning the other way at 70 mph. Since the engines are powerful enough to propel the plane to 140 miles per hour, they would be powerful enough to keep the plane moving at 70 mph, even though the ground underneath the plane is moving 70 mph the other direction.
Quote:
|
Originally Posted by Atooraya
there are plane sitting on ski's, but you can't put that situation and compare it with this.
but the problem is that the question first stated wasn't exact enough. people, and you, are saying that the airplane has "infinite thrust" and could push the airplane to whatever speed you want it to go to .
|
no, I am saying the plane only has enough thrust to move the plane at 140 mph.
Quote:
|
Originally Posted by Atooraya
ok
pretend that the airplane is rated for FASTEST at 140mph.
pretend it's on those ski's and the belt or whatever it's on is moving at 140mph in the opposite direction.
|
Okay, the plane wouldn't be moving then because the ski's would be dragging on the belt at 140 mph balancing out the engine's thrust.
But if the plane is stopped, then the belt would stop.
The equilibrium of the drag from the ski's and the thrust of the engine would be at 70 mph.
the drag from the ski's would then be equivelent to if the plane was moving at 140 mph on a nonmoving runway. 70mph forward speed of the plane + 70 mph backward speed of the conveyor belt = 140 mph's worth of drag balancing out the thrust of the engine.
anyways, the point is is that if the belt only spins at the speed the plane is moving, the engines thrust could keep the plane moving. not just relative the the belt... it would keep moving relative to the air (or the controll tower or whatever)
That is all there is to it. The thing about the question is that it is a trick question. It makes you think that the plane is stationary, when it is actually moving
cliffs: The wheels just spin at twice the speed the plane is moving. If the plane moves down the runway at 70 mph, then the runway would spin in the opposite direction at 70 mph. The plane would continue moving at 70, but the wheels would spin at 140 mph. (70 mph airplane speed +70 mph runway speed)