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  1. Join Date
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    #131
    Quote Originally Posted by mbeige View Post
    That was the initial premise, there is no error there.



    Like I said, my perspective does not include thrust.



    No thanks, I'm not paying hard earned money just to fly back there for this little demonstration. Like I said, I have already accepted your side because it's a matter of perspective. It's your turn to accept my side, because both sides (as demonstrated by numerous members siding with one or the other) can happen, because it's a matter of perspective.
    All right I was wrong. The error is in your understanding of the initial premise. Nowhere was it stated that the speed of the runway counteracts with the speed of the plane. The word is match. A world of a difference.

    Also, this isn't a matter of perspective because this is a scientific debate, not a religious or a political one. We're talking about facts, scientific laws, some logic here and there. It's not a matter of perspective, this is a simple yes or no question with just one answer. Everything here can be answered theoretically equipped with tools from high school physics (or maybe some basic college physics). I've already explained my side enough, and even went in-depth when some more tools were introduced such as kinyo's FBD.

    It can also be proven easily in the real world (using model planes and a moving platform of course). With the resources of say, Mythbusters, this question is a piece of cake to answer (unfortunately, they couldn't be bothered).

    I've explained my side enough. I don't think I need to introduce anything new. We're just repeating each other's arguments anyway.

    *beachbum: the car can't move forward on the dyno, but it can on the conveyor belt if the engine allows the wheels to spin fast enough. I've explained how it works for the plane already and why the plane doesn't even need much more force at all (unlike the car).

    OT: *OTEP, please just answer the question!! Have mercy!!
    Last edited by Alpha_One; November 10th, 2006 at 11:56 PM.

  2. Join Date
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    #132
    Will it matter if there were snakes on the plane?



    Honestly, hindi ko din alam ang sagot kaya itinanong ko dito sa forums.

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  3. Join Date
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    #133
    Yes, of course it would matter if there were snakes on the plane! Depende kung nasaan sila kung nasa cockpit mahirap na ata lumipad yung eroplano...

    Well, this material has been covered to death in a zillion Internet forums already. Aren't the existing explanations enough for you to decide?

  4. Join Date
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    #134
    Of course it's nice to see what Tsikoteers have to say about it.

    I've only seen the question in one forum. hehehe. I seldom venture out of Tsikot.com and Yahoo mail. :lol:

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  5. Join Date
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    #135
    I suggest you Google "airplane conveyor belt". Lots of sound explanations (avoid the forums, puro away lang dun sa mga yun) from people with lots of letters around their names.

  6. Join Date
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    #136
    http://mouser.org/log/archives/2006/02/001003.html

    Picked out some nicely written comments:

    Quote Originally Posted by MOUSER
    Alex: Perhaps this is subtle... there is friction between the tires and the conveyor, which is what makes the different between the wheels skidding along the surface and actually spinning. But there is also friction at the interface between the rotating wheel/axel elements of the wheel and the non-rotating elements. THIS interface is the source of any affect the conveyor might have on the acceleration of the aircraft.

    If the wheel/tarmac interface is infinitely frictiony, then the wheels won't slip. They will spin with a tangential velocity exactly matching that of the conveyor. However, if the axle bearing were friction-free, the wheel could spin as fast as it wants and have no affect at all on the aircraft velocity.

    It's the friction of the axle that is key in this question. How much force is imparted on the non-rotating elements of the aircraft by the action of spinning the wheels? Is that force sufficient to completely counteract the aircraft engines? It will have to be if you expect the aircraft not to move relative to the Earth.

    What I'm waiting to hear from anyone on the "stationary aircraft" side of this discussion is a description of how the axle bearing friction can possibly match up to the thrust of the engine(s).

    Remember that the engine thrust can essentially be thought of as constant, whereas the wheel bearing friction is going to be a function of wheel rotation rate. If the airplane is to actually *NEVER* move at all, then the wheel friction would have to counteract the engine thrust even when the aircraft has only just started to move. If the wheels turning at 1 RPM cause sufficient frictional drag on the aircraft to counteract the engines, then obviously wheeled vehicles would never have gotten off the drawing board. I think we can discount the "totally stationary" case as preposterous.

    So then, is there some non-linear effect that comes into play at higher speeds? I contend that there isn't. The wheel friction obviously doesn't cause any problems for planes taking off on normal runways, so at least up to the rotational speed necessary to allow the plane to take off, we can discount the wheel friction. The conveyor will never reach a speed faster than that of a plane at the moment of liftoff, so the wheel speed of a plane on the conveyor at the moment of takeoff would only be twice what is normal...

    So to prevent takeoff we must make the assertion that somewhere between v(takeoff) and 2*v(takeoff) a hugely non-linear friction effect takes hold and prevents the aircraft from accelerating.

    As far as I am aware no such effect exists.
    Quote Originally Posted by BOZO
    I've given up trying to make all my friends understand why the plane takes off, but this explanation worked with a few people:

    Imagine a plane flying three feet over a conveyor belt. The plane slowly extends its landing gear until a single, small wheel touches the belt. What happens to the plane? Does it come to a screeching halt? Or, does the plane continue in the air as the wheel spins really fast?

    Of course, the plane continues in the air as the wheel spins. The conveyor belt exerts no significant force on the plane.

    The "no takeoff" people must believe that my plane comes to a screeching halt as soon as the wheel touches the belt. But, that doesn't make intuitive sense to most of them.

  7. Join Date
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    #137
    Quote Originally Posted by Alpha_One View Post
    Besides, even a car or a man, which uses the friction between the feet/tires to propel forward, CAN move forward on a platform that matches it's speed backwards.
    It would be a different case if the plane is replaced by a car. If the road underneath moves against the car at exactly the same speed as the wheel rotates, then the car will not move. Now the FBD of the car wheel is different from the plane's case. For a car, there is no Ff to move the wheel forward. There is a torque that tends to rotate the wheel and there is Fb to move the wheel forward, so this time Fb is in the direction of the wheel's forward motion.

    Fb being frictional force is acting against the motion of the wheel's bottom surface which is moving backward, so Fb have forward direction. If the road is made to move in opposite direction exactly as the wheel moves, then Fb becomes zero, the wheel will not move horizontally, but the torque will continue to rotate the wheel.

    Just imagine this. Let's say the car is lifted from the road just an inch above road surface. Let the car's engine run for the wheel to rotate at some rpm equivalent to 50 kph. Looking at the bottom of the wheel, we will see that the bottom surface of the tire is moving at 50kph backwards. If the road is not moving, we will see that there is a diffrence of speed between the tire surface and the road, i.e., 50kph. If the car is dropped to a non moving-road at this time, it will immediately run at 50 kph being pushed forward by frictional force Fb.

    Now back to elevated car, wheel spinning for 50 kph. Let us move the road backward at 50 kph. We again look at the bottom of the wheel. We now find that there is no speed difference between the road and the tire surface. Both are moving backward at 50 kph. In this situation, if the car is dropped to the moving road, there will be no friction between the road and the tire because they are already both moving backward at the same speed. Fb will be zero, and the car will not move.

    So there is a difference if the vehicle is a car. On the plane's case, the engine's Ff is what make it move, and Fb is insignificant to oppose the plane's motion. In the car's case, it is Fb (in forward direction) that is forcing the car to move. With a moving road in backward direction, Fb can be eliminated down to zero, and car will not move. Although both plane and car do appear to be standing on frictionless surface, due to wheel bearings, the mechanism by which they are forced to move are different.

    The plane don't need Fb, so it does not matter whether the runaway moves or not. The car needs Fb, so it matters when the road moves. In fact, if the road moves at a faster rate than the wheel, Fb will act in backward direction and the car moves backward even if the tires are rotating for forward motion.

    So alpha_one, I am again in opposition to your statement. I'd sure be happy to be proven wrong again by you or anybody interested to reply ... although it would appear to be off-topic.
    Last edited by kinyo; November 11th, 2006 at 01:01 AM.

  8. Join Date
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    #138
    Quote Originally Posted by kinyo View Post
    It would be a different case if the plane is replaced by a car. If the road underneath moves against the car at exactly the same speed as the wheel rotates, then the car will not move. Now the FBD of the car wheel is different from the plane's case. For a car, there is no Ff to move the wheel forward. There is a torque that tends to rotate the wheel and there is Fb to move the wheel forward, so this time Fb is in the direction of the wheel's forward motion.

    Fb being frictional force is acting against the motion of the wheel's bottom surface which is moving backward, so Fb have forward direction. If the road is made to move in opposite direction exactly as the wheel moves, then Fb becomes zero, the wheel will not move horizontally, but the torque will continue to rotate the wheel.

    Just imagine this. Let's say the car is lifted from the road just an inch above road surface. Let the car's engine run for the wheel to rotate at some rpm equivalent to 50 kph. Looking at the bottom of the wheel, we will see that the bottom surface of the tire is moving at 50kph backwards. If the road is not moving, we will see that there is a diffrence of speed between the tire surface and the road, i.e., 50kph. If the car is dropped to a non moving-road at this time, it will immediately run at 50 kph being pushed forward by frictional force Fb.

    Now back to elevated car, wheel spinning for 50 kph. Let us move the road backward at 50 kph. We again look at the bottom of the wheel. We now find that there is no speed difference between the road and the tire surface. Both are moving backward at 50 kph. In this situation, if the car is dropped to the moving road, there will be no friction between the road and the tire because they are already both moving backward at the same speed. Fb will be zero, and the car will not move.

    So there is a difference if the vehicle is a car. On the plane's case, the engine's Ff is what make it move, and Fb is insignificant to oppose the plane's motion. In the car's case, it is Fb (in forward direction) that is forcing the car to move. With a moving road in backward direction, Fb can be eliminated down to zero, and car will not move. Although both plane and car do appear to be standing on frictionless surface, due to wheel bearings, the mechanism by which they are forced to move are different.

    The plane don't need Fb, so it does not matter whether the runaway moves or not. The car needs Fb, so it matters when the road moves. In fact, if the road moves at a faster rate than the wheel, Fb will act in backward direction and the car moves backward even if the tires are rotating for forward motion.

    So alpha_one, I am again in opposition to your statement. I'd sure be happy to be proven wrong again by you or anybody interested to reply.
    *Everybody: You're free to ignore my statement about the car and the one below. Suddenly I realize it's mostly irrelevant anyway.

    OT, *kinyo: For the sake of argument, assuming that both objects have to move in reference to the earth, and all the conveyor does is match the speed of the car relative to the earth, the car can still go it's normal speed. However, the engine would have to make the wheels spin faster than the conveyor can, i.e. the engine has to overcome the force of the conveyor. The difference is that the engine has to exert a LOT of force to overcome the conveyor, unlike the airplane problem where the engines probably won't notice a damn thing.

    As per your example, the car would indeed remain stationary (with respect to the rest of the earth) if it was standing on the platform doing 50kph backwards (Earth ref as well)and its wheels doing 50kph forwards. The car can do 50kph forwards, but (a big BUT) the wheels would have to rotate at 100kph forwards to catch up (requiring the engine to compensate). The relationship between conveyor, wheel, and car velocities (earth ref) remains the same as the situation with the plane, the huge difference lies within the forces they exert against one another (as you say, the forces are different, that is perfectly correct). Since the car has to rely on the ground for propulsion (Newton's third law) anything the ground does the car is directly affected by it. In the case of the plane, whatever force the ground exerts on it is almost negligible. However for the car's engine, it gets a share of Fb from the tires that's too big to ignore, but it's not bound by Ff=Fb either. The engine has to exert a greater force than Fb, and the car starts to move. That is what I originally meant with my post, if it wasn't too clear.

    Ergo, for the car to be able to run forwards at the same speed the conveyor is doing backwards, the engine must be able make the wheels spin *twice* the rate that would be used to run on a normal road. It doesn't matter what Fb or Ff is, except that the engine can make Ff>Fb. When this is the case, the "speed" (for a lack of a better term) of the wheel forwards would become greater than the speed of the ground backwards and the car starts to move forwards.

    To illustrate an example, let me mention again the hypothetical race-up-the-elevator experiment. I'm sure as children, at least those who grew up in the cities, many of you played around elevators, so I hope this would be intuitive.

    -You need two similar elevators next to each other, one going up one going down.
    -You and a friend are at the bottom. You're in front of the "down" elevator and a friend is standing in front of the "up" elevator.
    -At the go signal, both of you jump in the elevator.
    -Your friend just stands there, waiting to get to the top.
    -You must get to the top at exactly the same time as he does, or better.
    -Of course you can't just stand there, the elevator will push you over the ground and you trip off. You must walk or run really fast in order to stay with your friend/beat him. You have to pace MUCH faster than you would if you were climbing up a set of stairs.
    -Given exceptional balance and leg strength/endurance, you can match or beat your friend to the top.
    -After the experiment we have proved that you can go just as fast the platform you're standing on.
    -Remember, your friend's elevator is a similar model and goes up just about as fast as yours go down.
    -Since you either matched him or beat him to the top, you were able to go as fast or faster forwards than the elevator you're standing on is going backwards.
    -You'd be VERY tired however. (i.e. you've exerted lots of force and spent lots of energy)

    I hope that illustrates that a car (or a man), can indeed go as fast as, or faster than, forwards, as the platform he's standing on is running backwards. But unlike the airplane which has a wheelbearing to frictionally separate it from the runway, the car or the man would have to expend a tremendous amount of energy to overcome the force generated by the treadmill on the tires/feet. In the case of the car/man, the tires/feet are the ones responsible for propulsion thus the body is directly affected by the forces the ground exerts on it.

    P.S. I just wanted to illustrate that the situation (body moving forwards, platform moving backwards) is possible with the car/man, just insanely difficult. I hoped to explain that if, with athletic abilities (man)/a very powerful engine and perhaps even special gearing (car) the car/man can do it, it should be trivial for the plane to do so (which doesn't rely on the ground to be able to exert any force on itself, making whatever effect the ground has negligible).
    Last edited by Alpha_One; November 11th, 2006 at 01:57 AM. Reason: Clarifications

  9. Join Date
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    #139
    Quote Originally Posted by Alpha_One View Post
    OT, *kinyo: For the sake of argument, assuming that both objects have to move in reference to the earth, and all the conveyor does is match the speed of the car relative to the earth, the car can still go it's normal speed. However, the engine would have to make the wheels spin faster than the conveyor can, i.e. the engine has to overcome the force of the conveyor. The difference is that the engine has to exert a LOT of force to overcome the conveyor, unlike the airplane problem where the engines probably won't notice a damn thing.

    As per your example, the car would indeed remain stationary (with respect to the rest of the earth) if it was standing on the platform doing 50kph backwards (Earth ref as well)and its wheels doing 50kph forwards. The car can do 50kph forwards, but (a big BUT) the wheels would have to rotate at 100kph forwards to catch up (requiring the engine to compensate). The relationship between conveyor, wheel, and car velocities (earth ref) remains the same as the situation with the plane, the huge difference lies within the forces they exert against one another (as you say, the forces are different, that is perfectly correct). Since the car has to rely on the ground for propulsion (Newton's third law) anything the ground does the car is directly affected by it. In the case of the plane, whatever force the ground exerts on it is almost negligible. However for the car's engine, it gets a share of Fb from the tires that's too big to ignore, but it's not bound by Ff=Fb either. The engine has to exert a greater force than Fb, and the car starts to move. That is what I originally meant with my post, if it wasn't too clear.

    Ergo, for the car to be able to run forwards at the same speed the conveyor is doing backwards, the engine must be able make the wheels spin *twice* the rate that would be used to run on a normal road. It doesn't matter what Fb or Ff is, except that the engine can make Ff>Fb. When this is the case, the "speed" (for a lack of a better term) of the wheel forwards would become greater than the speed of the ground backwards and the car starts to move forwards.
    You agreeing on the car remaining stationary with respect to earth when the road matches the wheel speed is welcome.

    But mentioning Ff in the case of the car is irrelevant as there is no Ff on the FBD of its wheel. For the car's wheel, there is torque acting on the wheel which tends to rotate the wheel for forward motion if the wheel touches the ground. Torque will only rotate an object. It has no force to move an object to any direction. The car's wheel moves because Fb is developed by friction with the road. In a FBD, torque is represented by a semi-circular arrow, usually labelled "T". For forward motion to the right of this page, the arrow will be drawn in clockwise direction.

    However, you could possibly mean torque when you say Ff on the car, as you seem to equate Ff with Fb. This is fine with me. I would just make it clear that torque T is quite different from a linear force such as Ff. I'd repeat myself ... for an isolated body, torque T will make the object rotate but it won't move the object, while a force such as Ff acting thru the center (of gravity, such as the center of a wheel) of the object will not rotate the object but it will move the object to the direction of the force. If a force Ff acts off-center on a body, it will cause the body to rotate only enough to align the center-of-gravity with the force and as soon as alignment is completed, rotation stops but object will continue to move.

    Yes, for the car, it is possible to move forward or backward, depending on the speed difference between the wheel and the road. When their speed matches in opposing directions, the car remains stationary with respect to earth.

    I'd leave the elevator to others. Just reading it makes me tired.

  10. Join Date
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    #140
    Quote Originally Posted by kinyo View Post
    However, you could possibly mean torque when you say Ff on the car, as you seem to equate Ff with Fb. This is fine with me. I would just make it clear that torque T is quite different from a linear force such as Ff. I'd repeat myself ... for an isolated body, torque T will make the object rotate but it won't move the object, while a force such as Ff acting thru the center (of gravity, such as the center of a wheel) of the object will not rotate the object but it will move the object to the direction of the force. If a force Ff acts off-center on a body, it will cause the body to rotate only enough to align the center-of-gravity with the force and as soon as alignment is completed, rotation stops but object will continue to move.
    Torque, that's the correct word, thank you! No, I don't mean Ff=Fb, just that whatever force the car pushes the car backward the engine can overcome by generating enough torque at the wheels. Once it generates enough torque at the wheels to be able to convert enough forward linear force to overcome all the backwards linear forces, it's not necessary that the conveyor will be pushed back, because it will match the *car* speed not the *wheel* speed.

    The elevator race example is simple. It seeks to demonstrate that you *can* move across a moving platform that's moving in the opposite direction as you are, whatever it is that's propelling you. The platform, even if it's going backwards, still has a enough backwards resistance that allows you to push against it in order to move forward. The elevator isn't slipping back farther if you use enough force on your feet to get a net forward movement across an elevator moving backwards.
    Last edited by Alpha_One; November 11th, 2006 at 07:41 AM.

Will the Airplane Fly??????????