What is Digg?538 Comments
- Greg-J, on 10/12/2007, -18/+54.
PLEASE READ
After reading this entire thread, it's apparent that quite a few people are pretty clueless.
Aircraft do not derive their forward thrust at their wheels. If a plane is traveling at 250mph on a conveyor belt, and the conveyor belt is moving at 250mph in the opposite direction, the aircraft is still moving at 250mph. The wheels are spinning at 500mph. That's the only difference.
The friction caused by the wheels touching the conveyor belt isn't nearly significant enough to slow the craft down whatsoever.
But to those of you insistent that somehow the conveyor belt will magically remove the forward thrust coming from the propellers/jets, please see this link --> http://www.prntscrn.net/img.php?img=heres-your-sign.jpg
. - Floodle, on 10/22/2007, -6/+37yes it does, planes are not powered through their wheels like cars are, the engine fires up and blasts the plane forward, the conveyor belt spins in the opposite direction making the wheels spin at 500 mph when the plane is only travelling forward at 250 mph. The plane still takes off because it doesn't matter what the wheels are doing, they are simply spinning on their bearings.
- KSUdesigner, on 10/12/2007, -6/+20Read the article next time, you will see that it does indeed take off.
- inactive, on 10/12/2007, -4/+13I hope the people who commented above me have misread the question. Of course the plane would fly. A plane does not get it's airspeed from its wheels. If the plane is going "forward" at 200 Mph and the conveyer is going back 200Mph, then the wheels would be "spinning" at 400 mph.(*) But the plane is "still going forward!
Well, technically the wheels would be spinning at the same RPM they would if they were going 400 mph - xile, on 10/12/2007, -1/+9You guys crack me up. Jet airplanes fly by pushing air through their engines. This process is not impeded in any way by the conveyor belt because the wheels are not powered.
As a quick follow-up--why would the owner of a small plane tie it to the tarmac (assuming he/she was parked on a tarmac) when finished flying? The wind can blow the plane, cause lift, and flip it. The wheels didn't turn, but lift was created. - dawgma, on 10/12/2007, -2/+8Actually, it is all about the force - the force of the jet engines.
Regardless of how fast the conveyor is moving, and since the plane is not powered by its wheels, the plane will move forward proportional to the force of its engines. The wheels will spin as fast as the forward motion of the plane PLUS the speed of the converyor belt. All the converyor belt does is spin the wheels faster, without hindering the forward motion.
Don't worry Scuba, the answer must have been part of that small 5% of science you never could figure out. Ya. ;D - wrinkles, on 10/12/2007, -0/+6Look at it this way. Say you have rollerskates on a declined conveyer belt. Doesn't matter how fast that conveyer belt is spinning uphill, you are going to free fall down the ramp.
- ShaneApex, on 10/12/2007, -0/+6Consider this, take a toy car and tie a string to the front. Place it on a treadmill and pull it forward. pulling on the string represents the propulsion from the propeller, the wheels will spin as fast as they have to, but the toy car will move forward because there is a force pulling it in that direction. that is why the plane will take off. Now take a toy car with a motor and put it on the treadmill with the speed of the car and the treadmill equal, it will not move because the speed is powered by the force supplied to the wheels. That is the argument of the no flight people.
If this subject has taught us anything it's this,
Never offer advice,
A wise man won't need it,
And a fool won't heed it.
Just drop the subject, some people are to pigheaded to consider a thought that differs from their own. - geminitojanus, on 10/12/2007, -5/+11Sounds like a job for the Mythbusters. Go buy an old Boeing jet that's still flightworthy, build a "kite-rig" for it, build a gigantic conveyor belt, and let's try this out.
- jwolf, on 10/12/2007, -2/+8It absolutely can.
Because the plane relies on its engine to produce thrust which propels the plane forward, the speed of the wheels is irrelevant. Because the wheels are freewheeling and not transferring power to the ground (like a car or motorcycle), aside from the negligible amount of friction they produce do not really factor into the equation.
Assuming the fictional rolling runway is long enough for the airplane to take off, it will. - inactive, on 10/12/2007, -15/+21the lift is produced by the pressure difference created by the rapidly moving air over the wings, nothing else.... so without the rapidly moving air... ZERO and i mean this: ZERO takeoff... it's not going to happen, not only that but the plain actually has no momentum, so what's it going to do make a verticle takeoff?
only if it was a plane that was cabable of making a verticle takeoff...
the trick with this question is that the conveyer belt isn't actually counteracting the forward momentum of the plane, it's simply spinning the wheels while the jets push it forward..... - redcard, on 10/12/2007, -1/+6An airplane has four different factors acting on it. Lift vs Gravity, and Thrust vs Drag. Lift and Gravity are up and down vectors respectively. Thrust and drag are left and right vectors, respectively.
Now, the wheels on a plane free spin. And with the runway going BACKWARDS, the wheels will go FORWARD. So if the wheels are frictionless (which is what happens when you basically have them spinning at the ground speed of the thrust) , you're not providing any drag. You're not. Further, the wheels are spinning in exactly the same direction that the thrust is pushing. A lot of you are thinking that the wheels are going the opposite direction. Think about it. When you go forward, your wheels proceed in a counterclockwise direction when viewed from the front left side. The same occurs whether you're going forward or the road is going backwards. So.. going back to physics.
If you know something about physics, you know that the horizontal and vertical forces act independantly of each other. So lift vs gravity can and does occur seperately of thrust vs drag. I think we can agree on that.
I think we can also agree that if the forward acceleration remains constant, that the plane WILL take off.
So the question is, is the forward force of the engines equivellant to the backward force of the conveyor belt? The answer is no. It's not. How can I say that?
Remember earlier I said that the PLANE has four forces. Lift thrust drag gravity. Well, when you have the runway on a conveyor belt, spinning free spinning wheels, you're not acting on the plane. You're acting on the ground. Your energy is lost at a higher rate than the energy is lost from the engines. You're not as efficient pulling backwards as the plane is pushing forward because of the friction differences.
So we have acceleration in a positive direction. Acceleration is, of course, v/t/t. And if we have acceleration , we have an increase in velocity, and if we have an increase of velocity , we have forward movement. Thus.. lift is generated via bruenolli, and the plane flies. - inactive, on 10/12/2007, -0/+5There's no reason why the plane wouldn't take off, it's momentum doesn't come from the wheels, it comes from the jet engines, it would still move forward. Moving the wheels backward at twice the speed would do absolutely nothing.
- jonohull, on 10/12/2007, -0/+4Hey, crimson guy, maybe you need to go back to grade school. Look at my picture:http://img213.imageshack.us/img213/4779/deeplane0mh.jpg
I realize real-world conditions would give the bearings some frictrion. OK, so lets add 1 to the backwards force of the wheels. The net force is still in the forwards direction. - rmassie, on 10/12/2007, -0/+5Take a toy car and place a piece of paper under it. Hold the car and move the paper. The car stays. Paper moves.
Now do it again and push the car forward while pulling the paper. Car moves forward.
Your finger is the jet engine, the wheels spin really fast but don't really matter. The plane moves, generates lift, lifts off.
Some of you could say it a bit nicer though. - Flipperbw, on 10/12/2007, -2/+7All the NO people are saying is:
"No lift = no flight"
All the YES people are saying is:
"The wheels just spin"
How about we address each other's responses? I have no idea myself, and the site is duggeffect. - KSUdesigner, on 10/12/2007, -7/+11You people need to read the article first.
- Lord_oftheTrons, on 10/12/2007, -2/+6Pretty cool and awkward problem. It would definitely still take off though. Unless you increased the friction in the wheels/bearings to a ridiculous amount, this thing would still get off the ground.
- cranium, on 10/12/2007, -0/+4This is basic physics, folks. C'mon.
The only force that the wheels put on the plane is in the y direction -- they hold it up.
The fact that the wheels have *bearings* is to keep the x direction forces to the minimum possible. That means that the movement of the conveyer belt just makes the wheels spin faster. The plane's mass together with the law of inertia means the plane will remain largely unaffected by the conveyer belt speed.
The jet engines, on the other hand, are specifically for providing force in the x direction. Since f=ma the plane will accelerate in the x direction, providing the forward motion necessary to provide lift to the wings. - skyraider, on 10/12/2007, -0/+4"Unless the wheel bearings are COMPLETELY frictionless, the conveyer will be able to keep the plane stationary."
No - read the original link: the force applied to the plane by the conveyer is very negligible relative to the aircraft's thrust. The aircraft's thrust produces much more force than the conveyor belt, even though both are moving at the same speed. The two objects have the same speed, but different forces. - jonohull, on 10/12/2007, -1/+5Here is THE answer, hopefully easy enough for you guys to understand:
Let's assume the bearings have zero friction. Therefore, any force acting on the wheels in a backwards direction would not cause any movement to the plane, other than the rotation of the wheels. Now if the jets of the plane are started, there is a net force pushing the plane forward. Then, of course, there is air flowing over the wings, which generates lift.
Here is a crude drawing to back it up:
http://img213.imageshack.us/img213/4779/deeplane0mh.jpg - jonohull, on 10/12/2007, -0/+3Yeah, I thought that image would help, it's a LITTLE crude. I believe it gets the point across though. This is one of the worst comment wars I've seen in a while.
Dee plane! Dee plane! - blitzman, on 10/12/2007, -4/+8A jetliner normally takes off at about 160 mph. The conveyer would then be spinning the wheels at double the normal speed, or 320 mph. The jetliner won't take off because the tires will explode before it gets to that speed. If the pilot continued to gun the engines, a large fireball would be created as the undercarriage ground against the conveyer belt, or the pilot would lose directional control due to the uneven drag and it'll crash.
- geezusfreeek, on 10/12/2007, -0/+4I just lost all faith in the human race.
- machinelou, on 10/12/2007, -4/+8I can't believe anyone spends any time thinking about this question. The wheels on an airplane have no effect on speed caused by jet engines. Therefore, the plane will roll in the opposite direction of thrust (newton's law of motion) unless it has some serious wheel bearing problems (too much friction). The plane will more forward independent of some kind of super conveyor belt moving in the opposite direction. When the plane gets moving fast enough relative to the on coming air speed, the plane will lift off the ground. Again, this is independent of the planes speed relative to the ground. The plane only needs speed relative to the air. Ever seen pictures of a wind tunnel before?
- enisoc, on 10/12/2007, -1/+5As a university student, I can tell you with 20% certainty that the correct answer is most definitely C.
- manata, on 10/12/2007, -0/+4I'm clearly the only Digg user who hasn't commented on this yet...so here it is.
- cranium, on 10/12/2007, -1/+5Of course it will take off.
The planes wheels free-spin, they don't provide acceleration in either direction. The only important force is the thrust of the engines.
Since f=ma, f being the thrust, m being the mass of the plane, acceleration will be essentially the same as before. - Nevarius, on 10/12/2007, -0/+4At first glance it would seem the plane wouldn't move nor fly, but this would only apply if the forward momentum came from the wheels. Since the wheels are free rolling and the source of the forward movement (jet engines) comes in very little conflict with the movement of the conveyor belt it will take off. Look at it this way. If a plane was to land on this very same conveyor belt, would it just stop dead in its tracks once a wheel touches the belt. Considering the wheels are free rolling the only thing that would be adversely effected would be the wheels themselves
- sspooner, on 10/12/2007, -0/+4I fly hang gliders.
It's totally possible to fly with no ground speed at all. I do it all the time.
It's the air speed that counts. The air flowing over the wing produces lift, it's got nothing to do with how fast you travel over the ground.
The wheels do not drive the plane, they simply roll, so it does not matter how fast they roll, could be 500mph, could be zero, the plane still has thrust from it's engines (they don't know about the wheels) and hence it will fly it will move forward and hence it will fly.
Nice question though. - Grimgrin, on 10/12/2007, -0/+4The misleading part of this is "Increase the speed of the treadmill to match the speed of the plane". It gets people confused (I was confused myself) because most people read that as "Increase the speed of the treadmill to keep the plane stationary" And in that case the plane could not generate lift and would not take off.
The "trick" is that the plane's speed relative to the air is not tied to the speed of the wheels. Again Fnet = m * a.
Lets look at the forces on the plane
We have Fg for gravity opposed by the ground. We have Ft for the thrust of the engines. That's opposed by F friction on the ground and Fa which is the force that the treadmill exerts on the plane.
The argument is that increasing the speed of the treadmill to match the speed of the wheels does not transfer enough force to the plane to match the Ft of the engines.
I still dislike the way Cecil presents this though, because rather than addressing the basic error (the confusion caused by speed of wheels != speed of the plane in this special case) he and most of the other people who see the trick are either smug or outright dismissive of the people who don't see it. - gregm, on 10/12/2007, -2/+6THE PLANE IS PUSHING AGAINST THE AIR PEOPLE. THE WHEELS JUST HOLD IT UP!
Now, if the wheels are stuck to the treadmill, then ofcourse it's not going to take off. However, if they are free to roll, IT WILL take off!
The plane pushes against the air to provide its forward momentum. It doesnt use the ground except in landing to slow down. Even if the brakes are on, a plane can take off if it has enough power! - devoinregress, on 10/12/2007, -1/+5The wheels do nothing but reduce the friction between what the plane rests on and the plane. Te ground speed has absolutely nothing to do with the speed of the plain. If the plane were a car then it would get nowhere but the friction against the ground is insignificant.
if the ground moved under an airplane that was in the air what would happen? - iobuffa, on 10/12/2007, -0/+4http://www.grc.nasa.gov/WWW/K-12/airplane/move.html Relative Velocity - educate yourselves. Realize the plane does move. The question is worded to make you think otherwise. (Which makes it even more pointless if its intent is to deceive). Mention wheels and equal opposing speeds and people immediately think its stationary.
- rockintom99, on 10/12/2007, -1/+5Heh, yeah, at first i was totally at the assumption that it wouldnt take off, but then its really just a trick question. I DID smack my forehead, in the same way the article said. Good riddle.
- Greg-J, on 10/12/2007, -4/+7I know I already posted WHY the plane is not affected by the conveyer belt above, but I just can't help myself here.
To all the people who think ground forces have ANYTHING to do with aircraft momentum:
YOU ARE F*CKING RETARDED!
Bottom line. - kevbryant, on 10/12/2007, -1/+4take it in the converse..if i was falling, but the ground was always moving away from me, would i be considered as floating? of course not. this doesn't take much science, just common sense. flight is caused by air being moved around the wings,, if you were blowing intense amounts of air, say from a ridiculously huge fan in front of the plane, then yes this would work, but of course, the ground would have little to do with it.
- Comfuser, on 10/12/2007, -4/+7it would take off. just think physics, F=m*a. The forward force (from the thrust) in both case is always bigger than the backward force (due to the friction of the wheel and ground, and it's very small compare to the thrust force), therefore the net force is always forward. So the plane will accelerate forward in both cases and take off.
- inactive, on 10/12/2007, -13/+16The puzzle never mentions anything about "air". You have a plane being propelled forward by an engine, and you have a runway moving at the same speed as the plane. The wheels will spin twice as fast, but the plane picks up speed and then takes off. C%u2019mon, look harder at this.
- RobbieCrash, on 10/12/2007, -0/+3As a side note, it's really interesting to read all the pilots responses on here, for it seems we're the only ones who have the slightest ***** clue how this would work.
- pintong, on 10/12/2007, -2/+5Short answer:
If it was a car with wings, it couldn't because the treadmill would kill any speed it tried to gain.
If it is a plane, the thrusters will propel the plane foreward and the wheels spin with no help to the plane. - KSUdesigner, on 10/12/2007, -0/+3There's a key difference between the plane on a conveyor belt and you on a treadmill: your legs are pushing against the treadmill to push you forward, the wheels on a plane do not propel the thing forward, the engines do. Hence the plane does move forward, hence air flows across the wings, hence lift.
- SpeedyG, on 10/12/2007, -0/+3The drivetrain on a car is the method that a car uses to move forward. So on a car, the reverse treadmill works to stop the car, because the forward force of the car is applied solely through the wheels.
However, we're talking about a plane. A plane's "drivetrain" is its airborne engines, propeller or jet. The wheels on the plane are NOT what drives the plane. They're free-spinning wheels. In fact, the plane moving forward spins the wheels the same direction as the conveyor belt moving backward! - aviazn, on 10/12/2007, -0/+3But see, the force of the conveyor belt isn't acting on the plane, it's acting on the wheels, which can spin in place and will do no work on the plane.
- HeyBob, on 10/12/2007, -2/+5Comments on the Straight Dope article:
"If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH"
Bad math - the wheels rotate at 100MPH
Imagine the wheel and the surface of the conveyor belt as a set of gears - they both "move" at the same speed.
The speed of the plane through the air is 0mph (100 forward over the surface of the conveyor minus 100 backward of the conveyor)
Look at these cases:
Plane = 100 fwd on conveyor in opposite direction of 100 = wheel speed 100, air speed 0
Plane = 100 fwd on stopped conveyor = wheel speed 100, air speed 100
Plane = 100 fwd on conveyor in same direction of 100 = wheel speed 100, air speed 200
"imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you"
That changes experiment with a third force (the rope) not present in the original question (only 2 forces = rotation of conveyor and thrust of engines) and is invalid.
"The unwary tend to reason by analogy to a car on a conveyor belt"
The plane operates like a car until enough air passes over the wings to create lift. Only then does it become an airplane.
"the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air"
Here's your flaw - the plane moves forward reletive to the ground (the conveyor) but since the conveyor is going backwards by the same amount, it is not going through the air. - brianrlawson, on 10/12/2007, -0/+3I agree. If I'm standing next to this huge conveyor belt watching all of this nonsense go down, I'm watching a plane that doesn't move relative to my position. I just sit and watch a plane roll really, really fast on a treadmill and the engines help it keep pace with the treadmill. Since it isn't moving relative to me, a stationary object, it is stationary. If I had hair, I wouldn't feel it blowing because I'm stationary, just like the damn plane. That isn't a good formula for lift on those wings. No air moving over the wings (because, in case I failed to mention, the plane is stationary), no lift, no flying, no peanuts, no seat belts, no in-flight movies, no annoying pilots telling you which window to look out to see the Grand Canyon.
- bryan8m, on 10/12/2007, -0/+3If this gets dugg then here's a backup:
http://www.shorttext.com/sugl - grittathh, on 10/12/2007, -1/+4Another explanation:
Imagine an airplane mounted with skis trying to take off from a frictionless frozen lake. Can the plane take off in this case? Yes, of course.
Now, replace the skis and frictionless lake with the wheels and the conveyor belt. It doesn't matter if the conveyor belt speed matches the wheel speed exactly, they just create a frictionless surface. - six6to8eight, on 10/12/2007, -0/+3I'm pretty sure the answer is yes.
For the sake of simplicity, assume that the wheels are virtually frictionless (that is, they can spin completely independently of the aircraft and not transfer any force into the aircraft-- this is reasonable to assume because the thrust provided by the jet engine/propeller will easily be able to overcome any friction caused by the wheels).
Imagine now that the conveyor belt starts moving while the plane is stationary and in neutral (I'm not sure if planes have gears, but you get the idea). Neglecting friction, the plane will NOT move. It will sit there as the wheels spin backwards. If the belt moving backwards as the wheels spin freely doesn't move the plane backwards AT ALL (which, assuming my hypothetical, it wouldn't), then imagine what actual force applied to the wheels by the plane would do: assuming friction is a neglectable force (which, when compared to the power of a jet engine/propeller, it is), the wheels would just spin twice as fast, the plane would move forwards, and would thus take off.
EVEN if you want to believe that the the forward spin applied to the wheels by the plane is cancelled out by the backwards movement of the conveyor belt -- that the conveyor belt going backwards and the wheels going forwards cancel out and keep the plane stationary --, the forward thrust provided by the jet engine/propeller will create a net force in the forward direction, allowing the plane to move forward.
The most important thing to keep in mind, and the thing which most people don't think of, is that the wheels are SEPARATE from the plane, and only provide a very small amount of friction which, for all intents and purposes, can be neglected. - aeiou, on 10/12/2007, -0/+3I think one of the main problems here that people are having is the wording of the question in the digg posting. Look at the real question in the article. The conveyor belt is accelerating at the same speed as the speed of the plane, not the plane's wheels. How is speed measured in an airplane? Not through the wheels- that would be useless in the air. They use a pitot tube to measure airspeed (the speed of the air moving over the wings). It also works the same way if you use gps to indicate ground speed in this situation. As the plane moves forward, the conveyor moves at the same speed as the pitot tube/gps indicates (ground speed would be relative to the ground, not the conveyor). This would cause the wheels to spin at double the speed of the plane, and assuming that the wheels didn't break, the airplane would accelerate and take off almost as normal. In other words, if the plane were standing still in comparison to the ground/air, it would not have any speed, and the tredmill would not be moving anyways.
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