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12/14/2006 Entry: "airplane question" Except for Josh no one got the airplane question correct. I asked the question in this thread on the unicyclist.com forums that I frequent and currently the poll is 26 to 27. It also sparked 11 pages of debate, despite the question being answered correctly on page one. So, I have re-considered my stance on the question. Apparently people see the world fundamentally differently and it's hard to change one's preconceptions as to how physics works. It even took awhile for me to work the question out with my Dad, and he's one of the smartest people I know. Anyway, here is the answer and why. If this isn't enough explanation feel free to aim me at Dreqan and I can go into it in more detail. The airplane WOULD take off. The airplane WOULD have forward momentum in relation to a third object (or the air). Air WOULD be flowing over the wings. The conveyor belt WOULD NOT affect the plane at all. Why? It's easier to answer this question by asking questions. First, why do some of you think that there would be no air over the wings? This assumes the plane would not be moving. Why would the plane not be moving? You assume that the conveyor belt counteracts forward momentum. HOW does the conveyor belt do this? You assume by pushing backwards on the wheels... but that is impossible. The conveyor belt cannot counteract forward momentum in this manner. The wheels turn freely just like if the plane was tied to a post located beside the conveyor belt. Some of you are getting a car and a plane mixed up. How do cars move forward? By pushing off against the ground with their wheels. How do planes move forward? By forcing air out the back of their jet engines (more or less by pushing off the air). What do the wheels on a plane do? Nothing but hold the plane up. Imagine a plane on skis taking off, or a plane on pontoons taking off up river. Both would work just fine. A plane on wheels is no different. So lets talk about relative speeds. You have a plane on a conveyor belt and plane on a regular runway. Both planes fire the engines with the same thrust - say enough to get the plane on the runway up to a speed of 250mph. You are a third observer. Relative to you both planes are going 250 mph. Relative to the runway the runway plane is going 250mph. Relative to the conveyor belt the conveyor plane is going 500mph. Air is going over the wings of both planes at the same speed. The only difference between the two planes is that the wheels of the conveyor plane are spinning twice as fast as the wheels of the plane on the runway. Get it? If not feel free to aim me. Many people are having trouble wrapping their heads around this concept. It's a fun mental exercise, I think.
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