How many of you idiots actually paid attention in school?
This was a physics qustion I heard during my senior year of HIGHSCHOOL. In the correct wording of the problem the treadmill matches the speed of the plane.
Now some idiot messed up the wording and changed plane to wheels.
If it is worded correctly it will take off.
The way the MB tested it they used the correct wording so it will take off.
Ohio Rulz wrote:
Since this one can be tested, I'm giving my best guess as to what would actually happen. What's yours?
I think the coolest aspect though is the impossible hypotheticals that can't be tested. What about an infinitely powerful treadmill that can instantly anticipate the acceleration of the wheel? How does the treadmill respond to change in the wheel speed when it determines the wheel speed? It's like all the weirdest parts of a real analysis class. Gentleman, we are looking deep into infinity, and it is an ugly place.
Ohio Rulz wrote:
How does the plane have "speed" if it aint moving?
The same way I can be running 8 mph on a treadmill and still not be moving.
slave of required education wrote:
Kevin Rosenberg wrote:Hi, I am a physicist.
The conveyor belt is irrelevant to the question. If I were teaching an introductory physics course, we would say that the wheels in this problem are "frictionless." However, the small amount of friction is still irrelevant to the huge amount of thrust produced by a jet engine. The jet engine of a plane is "pulling" on the air, which is irrelevant to the ground. Engineers design the wheels of a plane to have as little friction with the axle as possible, so that its main purpose is just to keep the plane up in the air. That being the case, the plane will simply turn its jet engines on, the huge thrust produced would quickly overcome the relatively minuscule amount of friction of the wheels, and the plane will take off normally.
K.R.
Ph.D. Physics, MIT, 1999
You're an idiot, you should not be teaching physics. No air is moving over the wings, which makes it impossible to have lift. You need to have air flowing over the wings in order for it to take off... you just made all that up about being a physicist, cus you're actually retarded and don't have a clue about science.
You're the idiot. Can't you read what he said? He said that the plane would move forward by pulling on the air. He said the wheels could be thought of as frictionless. It won't matter which way they are spinning, or how fast they are spinning. They are frictionless. The only thing that will make a difference is the engine pulling on the air, propelling it forward, forcing air over the wings, and a takeoff.
How long is the teadmill/conveyor? If infinite long or as long as a runway, the problem statement should clarify.
One can assume a conveyor proportial in size to the plane as a runner on a treadmill. Based on this, the plane will just crash off the end of the conveyor when the jets pull it forward.
The answer is easy, the plane shoots down the runway and takes off. The wheels are frictionless, or they have friction so low that it wouldn't factor in.
Think of it this way: If you are wearing a pair of roller skates holding on to the back of a moving truck. How hard is it to hold on at 5mph? How much force does it take to hold on at 50mph? Pretty much the same amount of force in your grip at both speeds will keep your hands on the truck. It doesn't require 10x more force to hold on to the truck at the higher speed. This is because the wheels are frictionless, it doesn't matter what speed they are moving.
The same will happen for the plane. The engine will pull at the air (much like your hands on the truck). The plane will be pulled forward (much like your body). The wheels will be along for the ride (much like the roller skate wheels), no matter what direction or speed they are spinning.
The wheels have some amount of friction to overcome, but this is negligible.
There are two very unequal forces pushing on the plane.
1. The engines pulling on the air, thrusting it forward.
2. The conveyor pushing on the wheels of the plane, pushing the plane backward.
1 >>>>>>>> 2
The wheels have very low friction, so their force is very small compared to the thrust of the engines.
The plane moves forward and takes off. Crank up that conveyor to 1000 miles per hour, it still wont matter.
Runningart2004 wrote:
I believe the point that people are missing is the type of conveyor belt being used. If you can set up a manual belt, similar to a manual treadmill, I don't see this plane taking off at all. It would be similar to a car on rollers. In this cirumstance the plane's engines would turn on and produce thrust and move the plane forward, but the wheels turning would then turn the manual conveyor belt. So you can't have a plane taking off without moving forward, but that's not the riddle.
This situation is not at all like a car on rollers. A car moves forward by applying a force to the ground through its tires. When on rollers, that force is applied to the rollers and they turn.
An airplane does not getting up to takeoff speed by turning its wheels. It pushes/pulls the air. If there is no resistance to turning the wheels (i.e. no friction), the manual treadmill belt wouldn't move at all. It would be like rolling a rolling pin over the treadmill.
It would be like rolling a rolling pin over the treadmill.
Best comparison yet. And best reason why the airplane would move forward no matter what the speed of the treadmill.
It would be like rolling a rolling pin over the treadmill.
Best comparison yet. And best reason why the airplane would move forward no matter what the speed of the treadmill.
Do you know any physics at all? Gravity is only acting in one direction, downward, which is countered by the normal force (Fn) upward. What is providing the opposing force to the jet engines? It will have to be equally strong to make the plane motionless. The treadmill doesn\'t count as an opposing force because its making the wheels spin, will hardly any frictional force. So if you can draw me a force diagram to explain this mysterious force I will be delighted.
K.R.
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Sorry Kevin, but not true. If the hypothetical treadmill can keep up with the huge amount of thrust there is still something called gravity keeping the plane on the ground and in contact with the treadmill and in turn motionless. You will have to remove gravity to infer frictionlessness (is that a word).\"
Do you know any physics at all? Gravity is only acting in one direction, downward, which is countered by the normal force (Fn) upward. What is providing the opposing force to the jet engines? It will have to be equally strong to make the plane motionless. The treadmill doesn\'t count as an opposing force because its making the wheels spin, will hardly any frictional force. So if you can draw me a force diagram to explain this mysterious force I will be delighted.
K.R.
Did no one watch the video I posted on page 8? It clearly shows that a plane on a treadmill would move forward and thus be able to take-off.
Grimlock wrote:
Did no one watch the video I posted on page 8? It clearly shows that a plane on a treadmill would move forward and thus be able to take-off.
Yeah, but we don't know how fast that little plane is going. For all we know the treadmill is going 10 mph and the plane is going 11 mph and that's why it moves forward.
Also, it's not clear to me how that plane moves forward in the first place. It doesn't have engines like a real plane. I can't be sure if this is a good analog.
It is well known that elite pilots use a special kind of air brake. They put down the landing gear and turn on special engines on the wheels that spin the wheels backwards at up to 1000mph.
This spinning slows down the airplane, sometimes so much that it causes planes to crash
If you still can't understand why the plane will be able to take off, try to think of another situation:
Imagine a normal car in a wind tunnel. The wind tunnel is blowing 100mph wind at the front end of the car. The car's engine is giving it enough force to move 100 mph.
Does the car move forward? Of course it does. THis is the same reason the plane takes off. Idiots.
Ohio Rulz wrote:
How does the plane have "speed" if it aint moving? It's not like a bike (speed measured by wheel rotation). If it isn't moving, then the speedometer = 0.
Your definition of speed is flawed.
Exactly! A plane's speed is not measured by how fast the wheels are spinning (like a car) - it is measured by how fast it is moving through the air. So, if the plane is not moving through the air than the speed of the treadmill = 0.
Im an aerospace major.
Since I just took my final on aerodynamics yesterday, this is still fresh in my mind and Ill put in my 2 cents.
First, lets relate this plane to our own running bodies.
The thrust from say, jet engines, is comparable to our leg muscles.
Both will exert JUST enough force/energy to keep the person/aircraft in the same location on the treadmill.
The thrust from the jet engine will keep the plane moving "forward" on the mill, but really it is in the exact same place, just as our legs keep us in the same exact place in the room (on the mill)
The thrust produced pushes the body of the aircraft forward, but a jet engine doesnt make things fly or lift up.- wings do!
Airflow OVER the wings (as so many have stated already) generates lift (low/high pressure blah blah).
Knowing a little bit about treadmills, I know that when I run on one, the air around me in the room is STAGNANT, smelly, and humid. There is no airflow blowing against me from any angle.
That plane can be sitting (and keeping up) with a treadmill spinning at Mach 5, but its not going to lift into the air. Basically, there is absolutley NO air circulation, activity, or velocity around the wings whatsoever. PERIOD.
The fact that there are 9 pages of postings about this is sad.
If this did work, the millitary would have started using this on aircraft carriers years ago. Just think about it.
