So, did yesterday’s question confuse you just as much as it confused me? I’ve got a few good suggestions during the day.
There is only one solution to the exercise, either the plane takes of or it doesn’t. To simplify things, both the tyres and the conveyer belt can handle the stress.
First, let’s examine the, to most people, obvious and only logic answer: The plane can’t take off. The reasoning is rather straight forward. Most of us have probably boarded an aircraft and flown from point A to point B. Lift is the force that keeps the plane in the air between point A and point B and lift can only be produced when air is moving over the wings. During take off, the plane accelerates on the runway until it has enough speed - and lift - to take off. In short; the plane has to have a certain forward speed relative to the air surrounding it to take off.
Now, our intuition tells us that to gain speed, the plane’s tyres have to move on the stationary ground. Again, most of us have experienced this first hand. An airplane with its tyres on a conveyer belt that is moving at the same speed, but in the opposite direction of the tyres, will of course never gain any speed at all. Most of us have been in similar situations. If you run a treadmill and you start at the middle of it you will never be able to reach the front if the treadmill always keeps the same speed as your feet, but in the opposite direction. You are stationary compared to the ground and will not gain any speed relative to the air around you. Another example is emission testing on cars. You drive your car onto those roller things and accelerate. If everything works all right, the car will be absolutely stationary compared to the ground, even if the wheels are moving.
Our, at least my, intuition and sense of logic say that the same will happen to the plane. Unfortunately, as I wrote yesterday, the great minds of the internet - and some of the commenters here - tell a different story. The next part of this entry is my attempt to convince myself that the plane really will take off and that the conveyer belt has little or no effect.
The big difference between your feet (or the car) and the aircraft is that the aircraft is pushed forward by a force not directly connected to the conveyer belt - the jet engines. The jet engines will push the aircraft forward by sucking the air over the wings, thus creating the necessary _air_speed and lift. The aircraft will move forward, even if the conveyer belt is moving in the opposite direction. Michael Buffington has an analogy that might help:
Picture this - you’re standing on a skateboard that is riding on a treadmill. One person is standing in front of the skateboard on firm ground, and the two of you are holding a rope. This person pulls on the rope that you’re holding so that the rope moves exactly an inch per second, advancing you forward. No matter what speed the treadmill is going, as long as that person maintains the same rate of pull, you’ll advance forward an inch per second. Your skateboard wheels might go faster or slower in relation to the speed of the belt, but you’ll pretty easily advance forward. Change the rope to a stick, and the conveyor belt can travel in either direction at either speed and be just as irrelevant.
One might argue that the conveyer belt will accelerate backwards to cancel the forward acceleration, but that would not matter as length of the rope between your friend and you will be shorter, meaning that you have moved forward. The bad thing here is that my brain doesn’t really grasp that this example has much effect on the aircraft problem as there is no rope for it to hang on to. Or maybe there is? An invisible rope of air molecules? The jet engines are the aircraft’s arms, which grabs the air molecule rope and pulls itself forward. This will increase the airspeed as air molecules are sucked over the wings. As discussed earlier the air flowing over the wings will produce lift, which in turn will allow the plane to take off.
Let’s take it one step further. Remove the tyres, strap the aircraft horizontally to a structure hanging above the ground from a crane and turn the aircraft into a rocket with a fat ass rocket engine. This might be to take it a little bit too far since the rocket engine doesn’t work in exactly the same way as a jet engine, but it’s similar - to some extent. Roar is a rocket scientist, so he might be able to bury this theory without much effort. Anyway, turn on the rocket engine, full power, and release the rocket from the structure. Its engines will push so much air that the rocket will fly. Even if it was stationary just a moment ago.
OK, I admit it, the rocket example sucked and has little or no relevance at all as the Plane-Will-Take-Off-Party argues that the plane will move forward and take off more or less normally and not take off standing still relative to the ground. But, hey, it was an honest try.
Even though I might have convinced myself that the plain will take off by now (sort of, or maybe not), I’d really like to see it in real life. Can someone please rig a runway with a huge conveyer belt and place an aircraft with a good pilot on it?
Now, this might not have been the best scientific explanation and it’s somewhat poorly written - mostly because I’m not a scientist and that English is not my mother tongue - but I hope you feel a little bit more enlightened. Or confused.
Welcome back tomorrow when I’ll try to get to the bottom of the murder of John F. Kennedy.
vegard at vegard dot netwith your input. You can also use any of the other points of contact listed on the About page.
Technicaly thats not entirely correct, the jet engine suck air into itself. Then compresses the air, add some fuel, ignite and let it stream out the backside at a greater speed thus invocing newtons 3. law. Any force leaving the engine will result in an force equal in magnitude, and opposite in direction. And that force will push the plane forward, causing air to flow over the wings, producing lift.
Newtons third law
Rocket engines are more like a controlled explosion, but the principle is the same. Push gas out will produce an oposite and equal force on the actual rocket.
the plane takes of.
picture yourself driving a car on a moving belt. like you said the car
remains stationary. the car uses the friction to the ground to move
forwards. and when the ground is moving equally fast backwards the car
then you brows e-bay and buy a huge jet engine from som old russian Mig
plane. you mount the jet engine on the back of your car. and fire it up.
like roar said. the jet engine creates movement by pushing gas out the
back of the engine. creating thrust forwards. since this thrust comes from
the interaction between the air and the jet engine* (not the jet engine
and the ground) the car will now move.
buy large enough wings and john wayne will be standing on your doorstep
within hours wanting to buy your batmobile.
for the belt to make the plane stand still, it would have to move
incredibly fast in the opposite direction. this so that the friction would
be large enough. and this again because the tyres on a plane are actually
there to create a near frictionless conection to the ground.
this blog entry on kottke.org is rather interesting. many views and such.
PS. i have to leave now. before i could proofread what i have written. so
forgive my errors.
The engines of a plane do not get their energy from the ground, they get it from "throwing" large amounts of gas backwards from the engine.
|2006-04-10 23:26 CET|