pb1977 Posted December 4, 2007 Share Posted December 4, 2007 Found this on another forum hey guys there is a huge debate over at another forum concerning this question... Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind. Can the plane take off? Just wanted to know what you guys thought about it. I say it's not taking off. Link to thread - http://www.airliners.net/discussions/tech_ops/read.main/136068/ Im also saying the plane isnt going anywhere. Link to comment Share on other sites More sharing options...
Tony Posted December 4, 2007 Share Posted December 4, 2007 Gaahhh!! Not that one! Wheels or no wheels, the aeroplane is moving forward because the thrust is completely unrelated to the wheels as propulsion is gained against the body of air, not the ground, runway, or giant rubber conveyor belt Aeroplanes don't rely on wheels for forward motion, therefore they are of no consequence. Link to comment Share on other sites More sharing options...
Cyclone Posted December 4, 2007 Share Posted December 4, 2007 Yep, Tony is right on that one. In theory the conveyor belt accelerates to infinity as it can never match the speed of the wheels once the jet engines start moving the plane. Link to comment Share on other sites More sharing options...
Cyclone Posted December 4, 2007 Share Posted December 4, 2007 (still) air will only resist the direction of travel, not the speed... but you are totally correct to say that "The friction from the train's floor will stop pushing him forward and he will start to decelerate" is nonsense Wind resistance is proportional to velocity, so technically the resistance does depend on speed and resists opposite to the vector of the velocity. (Speed has no vector, it's not a measurement with direction). Link to comment Share on other sites More sharing options...
Tony Posted December 4, 2007 Share Posted December 4, 2007 Err... how is wind resistance proportional to velocity? Or are we talking about parasitic drag which does increase with speed? Once we get up towards supersonic speeds there are some effects due to compression but not at 'jumping out of an aeroplane' speeds that I'm aware of. Link to comment Share on other sites More sharing options...
Phanerothyme Posted December 4, 2007 Share Posted December 4, 2007 Yep, Tony is right on that one. In theory the conveyor belt accelerates to infinity as it can never match the speed of the wheels once the jet engines start moving the plane. Hmm I now believe the aircraft explodes in a spectacular fireball shortly after moving off. Link to comment Share on other sites More sharing options...
Cyclone Posted December 4, 2007 Share Posted December 4, 2007 Err... how is wind resistance proportional to velocity? Or are we talking about parasitic drag which does increase with speed? Once we get up towards supersonic speeds there are some effects due to compression but not at 'jumping out of an aeroplane' speeds that I'm aware of. The force exerted by the wind increases from very little at low speeds to equal to gravity at about 120 mph (for a human shape), hence terminal velocity, the point where the resistance from the air matches 9.8 N/Kg, otherwise you'd just keep accelerating. So the resistance from air friction is directly proportional to velocity. Don't take my word for it though, check with The physics lab. Or any other physics site I suppose. Link to comment Share on other sites More sharing options...
cgksheff Posted December 4, 2007 Share Posted December 4, 2007 Well then I would experience a decelerating force due to wind resistance. Doesn't apply _inside_ the plane/train, unless it's an open carriage. I may have revised my earlier thoughts on this subject, but ....... .... If the jump was on an open flat-bed wagon, .. Link to comment Share on other sites More sharing options...
Cyclone Posted December 4, 2007 Share Posted December 4, 2007 If he was on an open flat bed then the wind resistance would greatly reduce his jump distance (and he'd probably end up landing on the road, unless it was a very long flatbed). Imagine the film scenes when the guy leans into the wind when walking on the roof of the train, if he jumped he'd slow down very quickly and the train wouldn't. Link to comment Share on other sites More sharing options...
*_ash_* Posted December 5, 2007 Author Share Posted December 5, 2007 This makes my head hurt,just forget all about it and catch the next train!!I also though you meant 'Jump the train' IE not pay your fare.I remember these kinda problems from school and I just switched off and stared through the window, no hope of me working this out:huh: I'm a bit daft at times, but I understand all the responses so far at least. It was really a devious plan to pull some intelligent posters to my threads. p.s I don't do trains, it's the car for me:thumbsup: which brings me to this..... Link to comment Share on other sites More sharing options...
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