oldtimer Posted July 9, 2015 Share Posted July 9, 2015 You totally don't understand thrust. Which leads to speed, which has nothing to do with wheels on the ground, or skids, or skates. But that's OK. Someday you will. Who is "you?" I'm not sure who was the recipient of your fine mini-tirade/snarky conclusion. I can see where there may be confusion, and thanks! It was in response to jo56steph74 (my apologies if I misspelled the name). Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Also to your also, why do we park in a driveway, and drive on a parkway? +++ Why do people point to their wrist when asking for the time and don't point at their crotch to ask for the bathroom? Why is night called 'after dark' when it's really after light and still dark? Why is the third hand on a clock called the second hand? Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Also to your also, why do we park in a driveway, and drive on a parkway? +++ Is there a difference between 'slow down' and 'slow up'? What about 'fat chance' and 'slim chance'? What was the IQ of the inventor of the IQ test? If Superman is so smart why does he wear his underpants over his pants? Quote Link to comment Share on other sites More sharing options...
Coytee Posted July 9, 2015 Author Share Posted July 9, 2015 Coytee bumped this thread. It's his all time favorite thread. Blame him. Sure.....blame me while I'm at work unable to defend myself. Just wait until I see your ear lobes again! That said, I'll man up. I bumped it. It is always funny seeing all the back/forth gymnastics that go on with it. I'll banish myself to 12 minutes of Marie. Only problem is....I might enjoy it. Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Also to your also, why do we park in a driveway, and drive on a parkway? +++ If firefighters fight fire and crime fighters fight crime, what do freedom fighters fight? If you don't like bigots, does that make you one? What does a bald person put for hair color on their driver's license? Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Also to your also, why do we park in a driveway, and drive on a parkway? +++ If you had three quarters, four dimes and four pennies in your pocket you would have $1.19. You would also have the largest number (11) and combination of coins possible without being able to provide change for a dollar. If you removed a fly's wings, would it be called a walk? How is it possible to have a civil war? 1 Quote Link to comment Share on other sites More sharing options...
Jeff Matthews Posted July 9, 2015 Share Posted July 9, 2015 Yer killin' me, man! Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Also to your also, why do we park in a driveway, and drive on a parkway? +++ Why do overlook and oversee mean opposite things? Why are a wise man and a wise guy opposites? Why is a person who plays the piano called a pianist, but a person who drives a race car not called a racist? Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 9, 2015 Share Posted July 9, 2015 (edited) The thrust of the engine(s) is on the air, the craft moves forward, lift happens and it takes off. Oh ffs. Put the rubber band propeller plane on the treadmill and watch it take off. There is lift because the wheels on the ground versus the treadmill mean nothing. The thrust of the engine(s) is on the air, the craft moves forward, lift happens and it takes off. I'm taking the bait again, and running the line out!!!! Whizzzzzzzzzzzzzzzz!!! Yes, and if the wheels were frictionless and free to spin (thus treadmill has zero effect), the trust generated by the prop would propel the craft forward, which would in turn result in lift being generated by the air flowing across the planes foil. Ah!!!, but friction is real, and the craft will therefore move in a direction opposite to that of it's intended design use. And if memory serves me correctly, this problem stated that the treadmill moves and accelerate with a force that constantly matches the forward thrust (but opposite in direction) generated by the airplanes engine, be it a Lycoming / McDonnell Douglas / Pratt Whittney / Slingshot / Rubberband / Squirrels / Hamsters / Gerbils or anything your imagination can dream up;...... and unless the pilot stands on the brakes, there will always exist zero net movement of the aircraft relative to the air around it. Doesn't matter the engine type, size, or output, forward thrust is always being negated by the rearward movement caused by the treadmill. Put your car on the same self-accelerating treadmill with the skinniest tires money can buy, place the transmission in neutral and kick-on the treadmill. What happens? Bingo!! You either get it, or you keep bumping this very informative and educational tread. cough!, cough! Edited July 9, 2015 by Gilbert Quote Link to comment Share on other sites More sharing options...
oldtimer Posted July 9, 2015 Share Posted July 9, 2015 (edited) 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. That is precisely why it is a poorly worded question. It boils down to airspeed vs. ground speed. If speed is measured by wheel speed, like a typical automotive speedometer, the conveyor will not keep the plane from taking off, because wheel speed is meaningless in regards to thrust. If it can track and counteract airspeed gained by thrust, well it would be one hell of a fast conveyor for such little relative friction. Notice the question simply says the conveyor tracks the speed of the plane in the opposite direction. It doesn't say how. I guess the point is merely matching speed in the opposite direction is not enough to counteract the relatively small amount of friction from the wheels. The conveyor would have to go much faster than that of the planes actual speed. Edited July 9, 2015 by oldtimer Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 9, 2015 Share Posted July 9, 2015 (edited) 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. That is precisely why it is a poorly worded question. It boils down to airspeed vs. ground speed. If speed is measured by wheel speed, like a typical automotive speedometer, the conveyor will not keep the plane from taking off, because wheel speed is meaningless in regards to thrust. If it can track and counteract airspeed gained by thrust, well it would be one hell of a fast conveyor for such little relative friction. Notice the question simply says the conveyor tracks the speed of the plane in the opposite direction. It doesn't say how. I guess the point is merely matching speed in the opposite direction is not enough to counteract the relatively small amount of friction from the wheels. The conveyor would have to go much faster than that of the planes actual speed. How the moveable runway is controlled to counter the planes movement has no bearing, that the runway moves at a negative speed that constantly matches the forward speed produced by trust is all that is necessary to consider, and more importantly, all that is requested to consider. The magnitude of the friction (for the wheel bearings or bushings, and at the tires' contact patch) is inconsequential, that friction is real and exist has everything to do with the plane not being able to exceed the speed of the moving runway that supports it. It does not matter if the plane is in the lightest STOL category ever invented (J3 Super Cub, I think), or an Antonov 225 Cargo Plane. So long as there is a net zero displacement between the planes body and the air directly in front of it, no lift generated. In order for the plane to lift off the runway, the air speed passing over the wings must exceed the design stall speed,...... which, for the J3, means that the body of the plane will have to move at a min. of 30mph faster than the speed of the moving runway, which by definition cannot happen. Edited July 9, 2015 by Gilbert Quote Link to comment Share on other sites More sharing options...
oldtimer Posted July 9, 2015 Share Posted July 9, 2015 (edited) It doesn't matter if the plane is in the lightest STOL category ever invented (J3 Super Cub, I think), or an Antonov 225 Cargo Plane. So long as there is zero net displacement between the planes body and the air directly in front of it, there is no condition of lift Dead horse take 1000. I think we all agree on the conditions necessary for lift off. It is all in how you interpret the question. Notice it does not state that the conveyor moves fast enough to counteract the thrust of the plane, only that it moves in the opposite direction at the speed of the plane. Friction does enter into it if you take that part literally. A conveyor moving backwards at 150mph is exactly that. It is not a conveyor moving backwards at a rate which negates the planes thrust. If you want to go "by definition" the conveyor would not move at all if the plane were not going forward at all---no speed. Therefore by definition the plane is moving at a certain speed and not standing still. I once had a professor who was a master at constructing test questions. No matter how well you knew the material their tests were such that you also had to be very good at taking tests. I mean they were difficult! They should have been teaching advanced testing instead of Botany. Edited July 9, 2015 by oldtimer Quote Link to comment Share on other sites More sharing options...
oldtimer Posted July 9, 2015 Share Posted July 9, 2015 The question is actually an interpretive logic problem more than a physics issue. We all agree on the physics required. I think. 1 Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 (edited) In order for the plane to lift off the runway, the air speed passing over the wings must exceed the design stall speed,...... which, for the J3, means that the body of the plane will have to move at a min. of 30mph faster than the speed of the moving runway, which by definition cannot happen. Exactly! The plane is not in a vacuum and all that is needed for the plane to take off is that lift has to be created where the air accelerates over the wing and generally creates lower pressure over the top of the wing and higher pressure underneath to achieve lift. Of course, an additional thread could be started to explain the situations how a wing with a curved upper side and straight lower side generates lift and how a wing that isn't shaped like that also can generate lift when using symmetrical wings where air travels the same distance over both the top and bottom of the wing. With Coytee's situation, the plane needs to "move" through the air regardless of what the conveyor is doing to create the needed acceleration of the air over the wing. Essentially, we need to look at the propulsion of the plane. A propeller will create lower pressure over the top of the wing and higher pressure underneath and "pull" or accelerate the wing through the air. A jet engine creates thrust where gas is burned by the engine and "pushing" or accelerating the wing forward through the air. Essentially, the conveyor belt would have to somehow affect the air surrounding the plane because that’s where the plane derives its thrust which accelerates it through the air, and the plane accelerating through the air is how it creates lift. There is an aspect of friction involved between the tires and conveyor belt; however, I agree with Dr Who in that with the conveyor belt moving the ground underneath the plane at an offsetting negative velocity to the plane's forward velocity would only affect the rotation speed of the tires by doubling it. I would sure like to have the tire sales account for that airline. If a person could remove the "conveyor belt" concept altogether and keep the plane still where the tires do not move at all, then increase the airspeed with a very powerful "fan" the plane would lift into the air. Ever watch a gull on the beach do the same trick? From anther perspective, if we put the plane on a stand and lift the tires off the ground, how fast would a person have to spin the tires for the plane to take off? What happens when a plane flying at 150 flies into a headwind of the same 150? Will the plane drop from the sky or stand still over the ground or fly perfectly fine? Finally, what happens if you drive at the speed of light and turn your head lights on? Edited July 9, 2015 by Fjd 1 Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 9, 2015 Share Posted July 9, 2015 (edited) only that it moves in the opposite direction at the speed of the plane. Friction does enter into it if you take that part literally. Actually it does, for me anyway. If there is zero friction at the wheels, then the planes' body will not move, and wheels spin freely with the runway. Adding wheel friction into the equation means the planes support moves in the same direction, at the same speed as the runway. Does any of this BS matter, nah, not one bit. Edited July 9, 2015 by Gilbert Quote Link to comment Share on other sites More sharing options...
Sancho Panza Posted July 9, 2015 Share Posted July 9, 2015 Good idea, Chad; cuts out anonymous bumping. When I bump an old thread, I give an ***OLD THREAD ALERT*** Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 9, 2015 Share Posted July 9, 2015 Good idea, Chad; cuts out anonymous bumping. When I bump an old thread, I give an ***OLD THREAD ALERT*** Paranoia, paranoia Old forum threads are coming to get me He don’t post, say you never met me Anonymous thread bump laying with moles Digging up old, finding the holes Hear the voices in my head, I swear these threads are snoring But if you're bored, these old threads are more boring The agony and the irony, they're killing me I'm not sick, but I'm not well And I'm so hot, cuz I'm in old-thread hell I'm not sick, but I'm not well And it's a sin, to anonymous bump this swell 1 Quote Link to comment Share on other sites More sharing options...
Jeff Matthews Posted July 10, 2015 Share Posted July 10, 2015 Finally, what happens if you drive at the speed of light and turn your head lights on? They project light which, from your perspective, appears to travel at the speed of light. The speed of light is always constant, irrespective of the relative speed of the observer. Weird, but that's what they say. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 10, 2015 Moderators Share Posted July 10, 2015 Finally, what happens if you drive at the speed of light and turn your head lights on? They project light which, from your perspective, appears to travel at the speed of light. The speed of light is always constant, irrespective of the relative speed of the observer. Weird, but that's what they say. I answered this already in another thread last week. As you approach the speed of light the law of special relativity holds true, but it colapses when it equals C. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 10, 2015 Moderators Share Posted July 10, 2015 The speed of light in your reference frame is, v = (c − u)/(1 − u/c). For any speed u less than c this gives v = c so the speed of light is the same for you. But if u = c the formula degenerates to zero divided by zero; a meaningless answer Quote Link to comment Share on other sites More sharing options...
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