Guest " " Posted November 17, 2006 Share Posted November 17, 2006 DrWho "Then the belt will start moving in the forward direction since the belt's speed is in the opposite direction of the motion of the plane." huh....did I miss something...did anyone say that the planes wheels was driving the belt? You have this all wrong. plane with engine not running, sitting on a surface, will travel with that surface. If belt is moving rearward....so will the plane. If plane is on a raft going down the river....so will the plane....if plane is on a flat bed goign down the high way...so will the plane. Take a toy car and put it on a sheet of paper...pull the paper towards you...toy car will travel with the paper. If belt is moving 500 mph and planes take off speed is 70 mph....a plane on a moving belt traveling rearward at 500 mph needs to over come that 500mph and then proceed the additional 70 mph, total of 570 mph before the plane can take off. Plane will be travling backwards with the belt until engine starts forward motion. Dr Who, I think you are just rattling my cage. Quote Link to comment Share on other sites More sharing options...
mas Posted November 17, 2006 Share Posted November 17, 2006 My God! you folks are still confused by this??? No wonder so many are debating the effects of the color of insulation on how a cable sounds![] Without any math or force diagrams, please stop for a second, and I think I can explain this. Everything that is going on 'beneath' the plane is moot. It doesn't matter is a million ants are turning the conveyor, that there are geraniums next to the belt, or that the belt is 3 miles wide and weighs 16.483 tons, or that the pilot is chewing gum while waving to his mom while watching Friends on the monitor or that that he is a Presbyterian in denial over being gay. It doesn't matter! And here is why! You are confusing relative frames of reference! (And we are all adding conditions not posited in the the original question!) All we are concerned with is with the TOTAL net effect of every variable upon the total resultant force on the plane. And as was stated, the total net velocity of the plane relative to the wind is that it is moving at 0 mph. The plane is not moving relative to the wind! And the only important factor for flight (assuming the correct design of the wings and control systems etc) is the relative difference in speed between the wings and the air! Relatively speaking: The air is moving at 0 mph, and the plane is moving at 0 mph! That is our starting point! And we only care about the final differential! This focuses only on the velocity of the wheels and the conveyor! But let's change our frame of reference from the conveyor and wheels to the plane wing and the air. Thus, in order to achieve flight, the plane requires additional force sufficient to accelerate the plane's velocity relative to the AIR such that the pressure differential on the wings due to air flow will generate sufficient lift - in other words, the plane simply requires additional force to make it move fast enough relative to the AIR to fly! (This also assumes that we move outside the realm that was originally posited in the question!) Just like it doesn't matter what the aircraft carrier is doing under a plane (basically, ignoring wind velocity, the only need for a catapult is to accelerate the plane ins a faster mannner due to the short runway!), or that the world is spinning, or any other crazy scenario you can posit. The base scenario specifies that the total relationship is in equilibrium. Thus the summed forces on the left side of the equation equal the summed forces on the right - and they effectively 'cancel' each other! I don't care if there are a million variables creating 100 units of force on one side and 10 variables on the other side of he equation creating 100 units of force. They effectively cancel! This is why when we throw a baseball or drive a car, we don't have to calculate the effect of the earth's rotation on our speed! It is why a cop can use radar or laser to measure your speed without worrying about the larger frame of reference with the world's motion or another planets motion or the rate of expansion or contraction of the universe!! We start at unity as we are moving in unity relative to our starting state! Thus we start from unity! And from unity it is only necessary to apply force necessary to accelerate the plane to a velocity relative to the air sufficient to create the lift necessary to fly the plane! Quote Link to comment Share on other sites More sharing options...
oldenough Posted November 17, 2006 Share Posted November 17, 2006 Mas, you expect people who still think that the plane will not fly to understand that!!??? You could have at least at the end of it given a yes/no. Quote Link to comment Share on other sites More sharing options...
DrWho Posted November 17, 2006 Share Posted November 17, 2006 And as was stated, the total net velocity of the plane relative to the wind is that it is moving at 0 mph. Where is that stated? Nowhere in the question does it state or imply that the plane isn't moving relative to the air after the plane hits the throttle...or are you just positing that as the starting frame of reference? Quote Link to comment Share on other sites More sharing options...
mas Posted November 17, 2006 Share Posted November 17, 2006 Here is the answer. In the starting state of equilibrium it will continue in equilibrium - assuming the forces continue! In other words, it will remain as it is in EQUILIBRIUM! It can fly IF additional force (beyond whatever is being used to maintain the equilibium) is applied to the plane so that the velocity of the plane relative to the wind becomes great enough to generate sufficient lift - and I have no way, given the info provided - to know how much force is necessary. Quote Link to comment Share on other sites More sharing options...
Andy W Posted November 17, 2006 Share Posted November 17, 2006 Take a toy car and put it on a sheet of paper...pull the paper towards you...toy car will travel with the paper. Pull faster. The car will stand still. Why? The paper acting through the wheels upon the car can not generate enough force to accelerate the car. F=ma. You don't have enough "F" to give you any "a". Now with your other hand, push the car away from you and pull the paper toward you. (get a longer piece of paper if you have to). Wow! The car moves away!! The engine pushes on the plane. The conveyor pulls on the wheels. Which one wins? (I know, I PROMISED, but you're so close to getting it now, and I couldn't help it!) If belt is moving 500 mph and planes take off speed is 70 mph....a plane on a moving belt traveling rearward at 500 mph needs to over come that 500mph and then proceed the additional 70 mph, total of 570 mph before the plane can take off. NO! The belt can only move as fast as the plane. So if the plane takes of at 70mph and the plane is going backward at 70mph, then starts the engine, it will be moving 140mph relative to the belt, but 70mph relative to the air, and it takes off. The force of the belt though the wheels upon the body of the plane is only slightly greater at 140mph as it was at 70mph, so the plane will easily over come the additional drag. Even if the belt did go 500mph, I still think the plane could easily take off (if the tires were rated for that speed []). Quote Link to comment Share on other sites More sharing options...
oldenough Posted November 17, 2006 Share Posted November 17, 2006 Nice one Andy, that should be the clincher. Oh maybe not, better not get too excited. Quote Link to comment Share on other sites More sharing options...
IndyKlipschFan Posted November 17, 2006 Share Posted November 17, 2006 where do you bury the survivors? Not now any survivors.... But eventually, all of them where their family wants them to be burried. LOL Quote Link to comment Share on other sites More sharing options...
PhilMays Posted November 17, 2006 Share Posted November 17, 2006 Captains Log: 11/17/2006 6:30 PM Gave up on finding 300' x 60' conveyor belt. Checked Big Lots and, NO GO. Just bought a used Rosanne Barr girddle off ebay which, if I take it up some, should surfice. I now have the squirrel problem. I hear that mexican jumping chinchilla's will do a better job anyway. Will purchase sevaral hundred of them. BIG PROBLEM. Only $4.73 left in budget for plane. I am forced to get a fairly new MIG on the black market leaving me with a few dollars for misc. expenses. I'm getting excited. Have not seen the wife, kids, and dog for a while now. I have shaved my head and am living in a cave. Changed my name to Colonel Walter E. Kurtz.. Must...continue...on..... Quote Link to comment Share on other sites More sharing options...
mas Posted November 17, 2006 Share Posted November 17, 2006 If I apply the additional jet thruster (which all of us forgot was on the plane), and the force is applied relative to the air, the plane can fly, as the plane's speed relative to the air can increase to a sufficient velocity adaquate to generate the necessary lift. I am no longer applying force to the wheels and I am no longer dependent upont the velocity differential between the wheels and the conveyor. I am applying force relative to the air. I can fly. Wave goodbye![] Quote Link to comment Share on other sites More sharing options...
Guest " " Posted November 17, 2006 Share Posted November 17, 2006 Here is the answer. In the starting state of equilibrium it will continue in equilibrium - assuming the forces continue! In other words, it will remain as it is in EQUILIBRIUM! It can fly IF additional force (beyond whatever is being used to maintain the equilibium) is applied to the plane so that the velocity of the plane relative to the wind becomes great enough to generate sufficient lift - and I have no way, given the info provided - to know how much force is necessary. Yes thats the answer......engines off...belt moving rearward...plane is moving rearward...engines on...if enough thrust is applied...equilibrium will be met....still no lift yet....additional thrust past the point of equilibrium equal to what is normally required for the wings to reach take off speed and planes is moving forward...plane will fly...if plane does not move forward...plane will not fly. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 17, 2006 Share Posted November 17, 2006 Can anyone from the "will not fly" brain trust explain how any horizontal force from the conveyor is transferred to the structure of the aircraft? If you can't answer that, you can't support your conclusion. Added: I found this video. It was simply done, but it just might be enough for some disbelievers to see the light. Amateur Home Video Demonstration Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted November 17, 2006 Share Posted November 17, 2006 It doesn't need to. The original question states that "a plane is standing on a movable runway (something like a conveyor). As the plane moves the conveyor moves but in the opposite direction. The conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction." The question here is the "...speed of the plane..." relative to what? If we're talking the speed of the plane relative to the *GROUND* then the plane will take off. As the plane begins to accelerate, it is moving slowly at first, and the belt is moving slowly beneath it. I have no problem with the plane gaining enough forward air speed to take off. By the time it does, the belt will be moving at Vlof in the opposite direction, the wheels will be spinning at 2 X Vlof, and the plane will fly merrily away thumbing its nose at us. However, suppose we mean the belt will move in opposition to the plane based on the plane's speed relative *TO THE BELT*. I tried to wrap my brain around this, and to be honest the mechanics are beyond me. I think the end result is that if the initial conditions are to be met (belt moving at exactly same speed as plane in opposite direction, relative to plane's speed on belt) the only possible scenario is that the plane is stationary relative to an external frame, and never moves from its initial starting place. If the plane were moving forward, however slowly, it would be going faster relative to the belt than the belt would be going relative to the ground - a violation of the stated conditions. How you could make that happen seems to involve rapid and continous acceleration of the belt such that the small amount of force transmitted through the tires to the airframe (and to create the angular momentum of the rotating tires *SOME* energy is transferred to the plane, even if the wheel bearing were perfectly frictionless) is sufficient to offset the engine's thrust. My brain hurts. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 17, 2006 Share Posted November 17, 2006 ................However, suppose we mean the belt will move in opposition to the plane based on the plane's speed relative *TO THE BELT*................. Rephrasing my question: How does the movement of the belt oppose the plane's forward motion? Where does the opposing force act on the plane? Quote Link to comment Share on other sites More sharing options...
D-MAN Posted November 17, 2006 Share Posted November 17, 2006 Airflow over the wings is what makes it fly, and with zero air speed over the wings, zero flight. DM Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 17, 2006 Share Posted November 17, 2006 Airflow over the wings is what makes it fly, and with zero air speed over the wings, zero flight. DM Why do you assume there is "zero air speed"? What force is exerted by the conveyor on the plane to impede the plane's forward motion? I have to go out for a while. So you have time to think. Quote Link to comment Share on other sites More sharing options...
DrWho Posted November 17, 2006 Share Posted November 17, 2006 ................However, suppose we mean the belt will move in opposition to the plane based on the plane's speed relative *TO THE BELT*................. Rephrasing my question: How does the movement of the belt oppose the plane's forward motion? Where does the opposing force act on the plane? The opposing force is in the paradox of how the question is stated... Scenario A: If you choose to take the frame of reference of an observor on the ground for determining the plane's speed and the conveyor's speed (assuming the air is motionless), then the plane will take off and the wheels will spin at twice the speed of takeoff. So say the plane is moving forward at 10mph relative to an observor on the ground. This means the treadmill is moving 10mph to the right relative to the observor on the ground. And from the plane's perspective, it is moving 20mph relative to the conveyor and will eventually take off. Scenario B: But if you take the frame of reference of a person on the plane - regardless of the force causing it, the wheels are spinning at 10mph (again, as seen by a person on the plane). Then from the perspective of the plane, the treadmill will be moving at 10mph in the other direction causing the plane to be stationary relative to the air. For the plane to move forward on the treadmill, it requires a wheel velocity greater than that on the treadmill, but this isn't allowed because the question says this will never be the case. Basically, the naysayers are arguing that the thrust on the plane is being limited to prevent its wheels from spinning faster than the speed of the conveyor. It's all a matter of which perspective you choose for determining the speed of the conveyor as it relates to the speed of the plane. I personally think it is a silly question because in Scenario A it takes off and in Scenario B it "CAN" take off (which is the question), but it's not going to until you change the limits imposed by the question. Ultimately, it truly is a stupid question. Coytee needs to have his Jubilees taken away and forced to sit in the corner in their place until his wife says he can come out [6] Quote Link to comment Share on other sites More sharing options...
Guest " " Posted November 17, 2006 Share Posted November 17, 2006 " Why do you assume there is "zero air speed"? What force is exerted by the conveyor on the plane to impede the plane's forward motion?" Gravity is the force that results in the plane moving backwards on the belt. Take off speed needed to get the plane off the ground is the speed of the belt plus the normal take off speed. If the belt is moving rearward at 100 miles an hour, so is the plane. Plane must over come the 100 mph rearward motion just to be a zero mph. Add to that the normal take off speed. When the plane moves backwards on the belt, the wheels do not spin util an additional forward force enters the picture. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 17, 2006 Share Posted November 17, 2006 Scenario B: But if you take the frame of reference of a person on the plane - regardless of the force causing it, the wheels are spinning at 10mph (again, as seen by a person on the plane). Then from the perspective of the plane, the treadmill will be moving at 10mph in the other direction causing the plane to be stationary relative to the air. For the plane to move forward on the treadmill, it requires a wheel velocity greater than that on the treadmill, but this isn't allowed because the question says this will never be the case. Basically, the naysayers are arguing that the thrust on the plane is being limited to prevent its wheels from spinning faster than the speed of the conveyor. Here is the original question, I don't see the restrictions on the wheel speed: a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. The major thing that's confusing people is the speed matching thing. I think that's part of the beauty of the question. People get hung up on that. They also get hung up on irrelevant points regarding aerodynamics. There is really nothing wrong with the question - it's very rare in life to encounter clear questions. It's important to able to sift through problems and not get hung up on irrelevancies. As I've said before, the speed of the conveyor is irrelevant - choose any conveyor speed you want (in the Newtonian sense). Two times faster than the plane, three times faster, or whatever - it just doesn't matter as long the tires don't explode. The action of the conveyor is not transferred to the body of the plane. The planes forward motion is isolated from the action of the conveyor by the wheel bearings. That's why I keep asking anyone to identify and explain how any horizontal force from the conveyor acts on the plane itself. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 17, 2006 Share Posted November 17, 2006 " Why do you assume there is "zero air speed"? What force is exerted by the conveyor on the plane to impede the plane's forward motion?" Gravity is the force that results in the plane moving backwards on the belt. Take off speed needed to get the plane off the ground is the speed of the belt plus the normal take off speed. If the belt is moving rearward at 100 miles an hour, so is the plane. Plane must over come the 100 mph rearward motion just to be a zero mph. Add to that the normal take off speed. When the plane moves backwards on the belt, the wheels do not spin util an additional forward force enters the picture. How does gravity exert a horizontal force? Quote Link to comment Share on other sites More sharing options...
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