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One of the best textbook questions I have ever seen

  1. Aug 23, 2008 #1
    I though that was a great question for a textbook that forces people to learn how to communicate effectively.
  2. jcsd
  3. Aug 23, 2008 #2


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    Or how to become great teachers.
  4. Aug 23, 2008 #3
    Also, people post questions on here that clearly don't have a technical background. People try to respond by showing a bunch of equations to show off how smart they are. The result the person asking the question has no idea what the hell the person is talking about.
  5. Aug 23, 2008 #4


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    In that case, I'd use a see-saw analogy, since many people have probably been on one and can relate to the tipping (pitch) if one moves beyond the fulcrum.
  6. Aug 23, 2008 #5
    Just tell them that if fat people sit in the back of the plane, the pilots will have to use maximum forward pressure just to avoid dropping out of the sky.
  7. Aug 23, 2008 #6
  8. Aug 23, 2008 #7


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    I would simply show them a ruler held in the middle and explain to them that forces acting on the air surfaces of a plane act to lift a plane through a theoretical point referred to as the neutral point and that holding a uniform ruler between your fingers at the 6" mark was like holding it at its neutral point.

    Then I would place a small weight like a quarter right in the middle and explain that even with additional weight over the neutral point - the center of mass being still over that neutral point - that the ruler rises and remains level. Then you can show both of the effects of moving the quarter forward of and backward to this neutral point and explain, how the pilot would need to compensate for these effects in flight when making maneuvers as a result of any displacement of the center of mass from the neutral point.
  9. Aug 23, 2008 #8
    Guys, the point of this thread was not for you to answer the question......

    By posting a solution you're missing the very point of the thread.
  10. Aug 23, 2008 #9
    What's a neutral point, and does it depend on elevator trim?
  11. Aug 23, 2008 #10


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    Maybe explain the purpose then. Apparently you didn't communicate it effectively.
  12. Aug 23, 2008 #11
    I'm a pilot, and I honstely cannot say that I've ever heard the term "neutral point". There is a "center of lift" which remains fixed, and the "center of gravity", which changes all the time depending on how the aircraft is loaded, or even while it is burning fuel.

    With respect to stabilility, the "center of gravity" should remain within allowable distances from a specified point.

    Elevator trim plays no factor in these calculations.
    Last edited: Aug 23, 2008
  13. Aug 23, 2008 #12


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    I think the center of lift and the neutral point are the same thing.
  14. Aug 23, 2008 #13
    Elevator trim changes the lift of the hoizontal stabilizer, so would change the center of lift, would it not?
  15. Aug 23, 2008 #14
    My interpretation:

    When you truly understand the material, you will be able to explain it to others in the simplest means possible. The question above makes the student think critically about the material so that it can be explained to a layperson. Most of the textbook questions out there have the student think very technically about the subject, so they can explain it to their peers in the field. However, by not thinking about it in a manner posed in the above question, they are missing the connection of the material to everyday examples. It is these examples which help to reinforce the learning of the material, and the ability to use it in other applications besides the ones given in technical questions found in textbooks.

    Was that at all coherent?
  16. Aug 23, 2008 #15


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    That is a really good question. I know a lot of science and engineering majors who I have class with that could not for the life of them begin to explain anything they know in simple terms accessible to someone who is not in the field.

    I think if more scientists had this skill there would be less misconceptions about science in the world.
  17. Aug 23, 2008 #16
    It takes more time when you use a human language (they are ambiguous and inefficient).
  18. Aug 23, 2008 #17
    "Center of lift" is with respect to the airfoils (wings), not the stabilizer/stabilator.
  19. Aug 23, 2008 #18
    Is it already in lay terms? Does "unstable" mean astable or isn't there enough stick to take the airplane to the pitch equilibrium position?

    I'm not a pilot, just a former hang glider pilot and former aeromodeler

    Edit: Sorry, I didn't see your post, Holocene.

    The total lift on an aircraft is due every surface, including the horizontal stabilizer. More down trim reduces the effective angle of attach of the horizontal stabilizer. For a self correcting pitch the horizontal stabilizer needs to have an effective angle of attack less than the wing--otherwise the pilot or autopilot are required as part of the corrective feedback. So it's unclear to me if this problem is about pilot induced pitch oscillation or running out of stick.
    Last edited: Aug 23, 2008
  20. Aug 23, 2008 #19


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    His grandmother wasn't a barmaid was she?
  21. Aug 23, 2008 #20
    Last edited: Aug 23, 2008
  22. Aug 24, 2008 #21
    I think you're smart enough to figure it out on your own given the information provided. :wink:

    Besides chasely got it right. Read his response.

    I'll expand more tomorrow morning. Bed time now.
    Last edited: Aug 24, 2008
  23. Aug 24, 2008 #22
    EDIT: ah! and I thought I would get the points! ... maybe next time haha

    was your point that it's a great textbook question because it puts the student outside of his safety zone and pushes him to think creatively and solve a familiar problem using an unfamiliar approach, which is a useful skill?

    I hope I got it right :biggrin: but I feel like I'm writing an essay.

    Isn't "science" that thing terrorist devil worshipers use to kill democracy?

    Worse than those who make it seem inaccessible I think are those who treat the audience like children.
    Here I think the problem is as much with the public as with the... actually mostly the public. People are more interested in the "magical" explanation than in the logical explanation. People love the spooky over-simplified twilight-zone explanations of things like (cue spooky music) quantum mechanics they get from movies like The Secret or other such crap.

    I doubt a series like Cosmos that doesn't treat the audience like a child would do as well today as it did back then, not unless it promises people that they can change the world with their minds. I don't know if you guys have noticed, but QM has become a new religion. All the quacks out there are using it as the foundation of their new everything-is-possible-you-are-the-master-of-the-universe religions. It's quite sickening.

    Just rent "The Secret", or worse: "What The Bleep Do We Know." (or download them because those movies don't deserve a penny of your money.) They use people's ignorance on the subject, misquoting noted physicists (some of whom have sued) and misinterpreting science, as a means to manipulate them into their million-dollar sects.

    If the public were really interested in learning, then these hacks would not be appearing in the mainstream media all day, and instead they'd give more airtime to people like Alex Flippenko or Phil Plait who can, when given the chance, reach the general public ... so the media are at fault here too I guess.
  24. Aug 24, 2008 #23


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    I agree, this is a great question, but it's because it actually asks the student to do more than translate it into laymen's terms. It is really asking if the student fully understands the concepts enough to explain it in plain English rather than mindlessly playing with equations.
  25. Aug 24, 2008 #24
    I'm with Holocene, see a lot of mixup. Anyway if I tried to to explain, I imagine the setting: Uncle-Andre-explains-it-to-nephew-Chris (10).

    Incidently anyone figured out why the aerodynamic center should be ahead of the centre of gravity? (with the exception of the F-16, Eurofighter, etc)
  26. Aug 24, 2008 #25
    Q: You have come across a really good question in a text book. You are asked to present this question to a bunch of physics nerds. You need to present the question as an example to a way of asking questions, not as a question to be answered. Assume that the nerds are educated and anal about proving how intelligent they are. Write an example of how you could accomplish this.
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