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箭的尾部为什么要插入羽毛? 第1页

  

user avatar   xi-men-tai-lang 网友的相关建议: 
      

巧了,我今天刚做了个实验。


我今天早上收拾院子发现了一根很好看的树枝,心血来潮就想做把弓箭,于是我花了一上午的时间做了这把弓。


工艺很糙,因为毕竟第一次做木工,弓的制作组也是上网现瞥了一眼,于是就刀刮了一遍木头,砂纸又磨了一遍,弓两头的凹槽只砍了粗的那头,另一边直接用绳子绑在小枝上也不会乱动,就没挖凹槽。

也不敢用火烤,怕把院子点了,于是就手工扳造型,差不多扳成合格的形状,就勒弓弦了。

我怕弹性不足,就按着复合的思路,在中央部位前段紧紧绑上两根一次性筷子,用的是装箱用的塑料绳,贴合得相当严密。之后用纱布缠好握把位置就完成了。

拉了拉,不是很紧,但是弓身弹性是够的。于是开始想怎么做箭。

最后我从小区路边找了两根够长够直的树枝,把枝干和底部磨平,冲刷晒干,大概一顿饭的功夫就弄完了。

其实说直也没那么直,不过好歹不是烂木头,挺实在的,而且打磨得也挺光滑了,就差个箭羽,想着射出去效果应该还不错吧。

https://www.zhihu.com/video/1133767479796121600

虽然料到不存在穿透力和稳定性,但是这个真的是歪的可以,基本刚出手就横着了。

其实本来我就玩玩觉得无所谓,但是一躺下看知乎就看到这道题(知乎监控真可怕)。

于是我就想,是不是加个尾羽会好一点?可是我家没有羽毛呀!不过这个难不倒我…


我把家里的开锁公司名片给撕了,于是就有了如上图的箭羽。

那射出去什么样呢?

可以看到比上次稳了点,但是还不够,我就寻思是这个尾羽太散漫了吗?要不黏在一起?

然后射出去效果很差,总是打弓把,于是我干脆改姿势,把箭搭在外侧:


https://www.zhihu.com/video/1199951738192789504

可以看到,第一次被吹飞,第二次打手,第三次改了姿势才射出去(其实还有更多次没录的都跟一二次一个情况),不过第三次明显弹道笔直。

后面我就想,这玩意得从原理入手啊,尾羽不就是一个定风舵加一个自旋效果吗?不如我再改装一下让它自旋起来:

…没办法手工就是这么糙…不过基础构造对了应该就没问题吧…

于是试出来是这样的:

https://www.zhihu.com/video/1133778380108431360

你看到了吧,完全的水平,虽然因为弓箭材料问题加上我手打滑,射速并不快,但是很明显拥有四尾羽后稳定性大大提升。


最后,我一定要声明,我可是个守法公民哦,该弓箭不存在任何杀伤力各位都看见了哦!真的是纯粹为了科学哦!


user avatar   han-da-gou-8-75 网友的相关建议: 
      

我相信,最初的箭是没有箭羽的。只是一根磨尖的,光秃秃的树棍。


但随着使用率的普及,人们渐渐发现一个问题:箭射出超过10米后准头就没得看了。最后射到哪完全看缘分。绝大多数人认为这种武器就这样,大家都这么用。只有少数人会纠结“为什么10米内有点准头,10米开外就成了随机分布了?”


这些人当中有那么几个脑袋灵光的,开始研究关于箭飞行的原理。最终有人发现一个规律:射箭时顺风射的远,逆风射的近。为什么会这样?有人开始认真思考箭与风的关系。


没错,空气跟水流一样!也是有阻力的!朴素空气动力学诞生。


古人没有高速摄影机,也没有风洞。要发现风的秘密并不是一件容易事。(实际上,现代人想理解也并不容易)所以我个人推测:箭羽实际上是在一个偶然的情况下诞生的。可能是从鱼的尾鳍中获得灵感,也可能是从飞舞的树叶中获得灵感。总之,人们发明了箭羽。


箭羽的原理,本质上是利用了杠杆原理和空气阻力实现的“不倒翁式”风阻修正。其实做个示意动画就一目了然了,但做这东西太麻烦,我就尽量用语言表达出来。


找一张长纸条。平放面前,用手捏住一端,使劲吹气。你会发现长纸条被你笔直的吹起来了。


很多人知道这个现象,但未必知道原理。如果用高速摄像机拍下你吹气的过程,你就会发现纸条不是从疲软态一下被你吹硬的。而是剧烈的上下抖动,幅度越来越小,频率越来越快。最终保持在一个极高频率极小振幅的稳定状态。怎么样?是不是有点像波长的特征?跑题了,箭羽的原理就是如此。


一只完美的箭,需要将重心调整至箭的中心偏前。箭头要有流线造型降低风阻,尾端设置箭羽用来在“特定情况”下提升风阻。


那什么是特定情况呢?气流笔直向后的时候当然不算特定情况。因为绝对垂直下箭羽不起作用(且自然环境下气候和工艺也达不到绝对垂直)。只有当气流不稳定,上下左右倾斜的时候,箭羽才会产生对应大小的风阻,给箭一个反作用力。这样箭就不会被吹的越跑越歪最终失控,而是疯狂左右打轮逐渐稳定。就像你大力推一个不倒翁,通过重心的杠杆作用,引力也会给你一个同样大小的反作用力晃回去。


说的再通俗点:箭羽就是利用风阻,将整支箭变成不倒翁。一边摇晃一边前进的。虽然因为风阻牺牲了一点动能,但提升了弹道稳定性。要知道箭只有笔直插入的时候动能传递最有效率。稍微斜一点效果就如同倾斜装甲跳弹了。


有人可能会问:一支箭也就四片甚至三片箭羽,但气流的方向可是矢量的,有无穷个方向。四个方向兜不住啊。


你算是问到点子上了。而且古人也发现了这个问题且解决了。解决方案就是做两种箭羽。如果对精度没那么高要求,就用直羽。精度差动能损失小。(其实这是主流需求)如果你是强迫症患者,就用螺旋羽。利用风阻让箭在飞行时旋转起来,这样就能完美兜住各种方向的气流了。但代价是动能损失很大,射程短侵彻力小。


至于制作工艺,参考其他答案。


user avatar   huaxiaoji-29-37-70 网友的相关建议: 
      

全面地,从宏观介绍差异的一篇文章:

12 Differences Between Chinese Education and American Education

Posted on June 1, 2007

This analysis is being prepared for a presentation I am going to make at the International Conference on Intercultural Education in Harbin, China on June 22-24. I would be interested in receiving your observations, comments, questions about the differences between Chinese and American educational practices.

  1. Class Size is the first noticeable difference at the middle school and high school level. Teachers typically teach two classes (in an 8 period day) with from 55 to 65 students. American secondary teachers typically teach five or six classes with class sizes ranging from 25 to 30. The Chinese teachers use their non-teaching time to grade papers and to prepare for their classes, except for the head teachers (banzhuren) which will be explained later.
  2. While Americans think the "cohort" concept is innovative, China has been using it for decades. Throughout China, students beginning in a school are put into classes and they stay in those classes with one another for the entire time they are in that school unless higher test scores permit them to move to a more advanced grouping. Understood in the Chinese system is that this group of students will learn each of their subjects together. In America, students are not grouped into such classes. Instead, the 30 students who are together for English class will be randomly split up into any of the other subjects for the next period and the period after that, and so on. The next year, the students are totally mixed up again into different classes. Occasionally, the same class of students will take two courses together, such as English and history, but that is rare. The Chinese carry the cohort concept into the university level as well. My four classes of students stay together for all of their required courses the whole time they are at university.
  3. Chinese students stay in the same classroom for their main classes and their teachers come to them while American students change rooms every period and the room belongs to the teacher. Thus, Chinese students don’t have hallway lockers. Students sit in the same seats for each subject and keep their materials in a shelf under their desk top. Many students have cloth covers for their desk and other means of making it "homey".
  4. Chinese education is built on what Americans call "looping". The teachers of the students in the entering class will also follow their same students to the next grade level and the next. In America, it is very unusual for teachers to move with their students from one grade level to the next at the middle school or high school level let alone to loop for the entire period of time the student is in that school. At the primary school level, students begin in grade one with a teacher and stay with that teacher every year they are in primary school. My university students reflect on that teacher as being so very important to them that they really didn’t want to leave them when it was time to go to middle school. American teachers, on the other hand, tend to specialize in the curriculum and content for a particular grade level and then stay at that level. Sometimes, teachers who want to teach older students will ask to move to a higher grade, but then that teacher would typically stay at that level until retirement. That practice means that in America, subject matter and teacher preference might be valued more highly than student needs or student learning.
  5. Another significant structural difference between American and Chinese schools is the concept of head teacher or "banzhuren". The banzhuren takes additional responsibility in delivering instruction, supervising their specific class of students, and in knowing their students and the families of the students and in communicating with those families. For less than 200 yuan per month more, the banzhuren will arrive at school prior to 7:00 a.m. to prepare for the day and to work with early arriving students. The student day at the middle school ends at 4:55 and the teachers leave shortly after that. The banzhuren will not only teach her specific class that she is in charge of but will also sit in on many other subjects throughout the day so she can monitor the progress of her students with other teachers, counsel her students, and contact the parents of those students if necessary. The banzhuren will monitor her class during lunch and nap with them after lunch. One banzhuren told me that she is like a mother to those students who don’t have the parental support they need. In addition, at Liaoning Normal University Junior Middle School (LNUJMS) thebanzhurens are expected to visit the homes and families of half their students sometime during the first term and the other half during the second term. These visitations would take place on Saturdays or Sundays or on holidays. The banzhurenwill, after three years, receive a bonus based on the academic improvement of her class. In America, the individual teacher is expected to make parent contact when a student misbehaves or is not performing at a satisfactory level. In American secondary schools there is also a person called a guidance counsellor who will assist with parent contact. However, the guidance counsellor will have a case load of 350 to 500 students and she often must resort to group counselling sessions.
  6. The Chinese believe in merit pay and in using student test scores for teacher evaluation. For example, when the 9th graders leave middle school they are tested to see which high school they are qualified to attend. Since the same teachers have had those students for three years, they compare their entry score to their exit score. The classes that showed the most academic gain resulted in that banzhurenreceiving a bonus that might range from 3000 yuan to 6000 yuan (one month or two months pay). The same is true at the high school level. On the other hand, a teacher whose students did not show growth will be evaluated accordingly. At LNUJMS, the math team won first place in the District math contest. As a reward, the four math teachers split a 2000 yuan bonus. The teachers I have talked with like the bonus system. So for years the Chinese have been doing what the American conservatives have been advocating and our teacher unions have been fighting.
  7. Discipline in Chinese schools easier than in American schools. For example, at LNUJMS, I was surprised to find minimal student supervision during lunch and between classes. One administrator and one teacher were in the hallway and no teachers were in their classrooms. The other teachers had gone to their offices to meet with students for academic or disciplinary reasons or for a rest between classes. In America, the time between classes is as short as possible, three, four, or perhaps five, minutes. Students are expected to move from their classroom, go to their lockers to get materials for their next class, and then move to that class. The American teachers are expected to be in the hallways during passing periods because that is when fighting and other misbehaviors occur. It is nearly impossible to even imagine a 10 minute passing period in an American school.
  8. Chinese students are very respectful. When Chinese students recite, they stand; when students hand in a paper, they use both hands as if they were making a presentation of the paper to the teacher; when students refer to their teacher in writing, they often use terminology such as , “Our dear teacher.” In interviewing thebanzhurens, they commented that is their duty to teach students how to do well in life and how to be a man. University students, when asked to recall their middle school and high school years often speak of their teachers in very exalted ways telling how much their teachers meant to them.
  9. Chinese students play active and important roles (zhirisheng) in sweeping the classrooms, scrubbing the steps, serving meals, being class monitors, and helping teachers. Student monitors can be seen wearing special armbands in the hallway, watching to make certain students are doing their twice-daily eye exercises properly, providing leadership on the marching field, watering plants, empty bins, cleaning windows, helping to distribute the daily lunch, and so on. Students always seemed to be carrying out their tasks very seriously and in good humor. The student monitor system is utilized at the university level as well with these appointed students helping the teacher in making copies, distributing and collecting papers, contacting classmates and so on. Like the concept of banzhuren, the concept of zhirishengcannot find its English equivalent due to the different Sino-American educational systems. Most Chinese schools are operated on the zhirisheng system for the purpose of maintaining clean classrooms and schools.
  10. Chinese students buy their textbooks each year and the textbooks are soft cover and relatively thin. The textbooks I looked at had a 2006 copyright and I understand that they all have recently undergone revision. The cost is about $1 for a textbook and $2 for a workbook that accompanies the textbook. Students make heavy use of highlighters and annotations in their books. In addition, the textbooks often have moral lessons built into them. For instance, when a particular scientist is highlighted, aspects of his/her life that are exemplary are extolled, such as hard work, protecting the environment, overcoming obstacles, etc. On the other hand, American textbooks are hard cover, updated every six years (at least in Indiana), and are rented to students. Students are not allowed to mark in their textbooks in America.
  11. It almost goes without saying that the curriculum in China is standard nation-wide and that students have few elective choices. National goals, national curriculum, national expectations, national exercises, and even a national class schedule are all built around the Chinese Education Schema that is based on centuries of tradition. In America, education is primarily the responsibility of the state governments. Counting Washington D.C., there are 51 separate governments that give direction to public schools. Further, except for Hawaii, states are divided into school districts that also have certain autonomies. Indiana, for example, has 292 school districts, 292 different teacher contracts, salary schedules, and sets of working conditions.
  12. Deeply embedded in the Chinese culture is the examination system. Since the Song Dynasty (960 AD), the Chinese have relied on the examination process to identify their governmental leaders. Now the examination system is used to determine which university students are able to attend. These national exams were given on June 7 and 8. Local middle schools were used as test sites and those middle school students had a two day holliday. The test is of such great significance that parents rent hotel rooms nearby so their student can have a quiet lunch and take a nap. I saw one hotel with a big banner wishing students luck on the exam. Students will take either the liberal arts test or the science test. For liberal arts students, the first test, Chinese, was from 9:00-11:30. Students were dismissed and came back for the math test from 3:00-5:00. On Friday, the schedule was the same, with the morning session being geography, politics, and history and the afternoon session being English. The parents filled the street in front of the school and anxiously awaited students coming from the test sites and wanted to know how their child did. Taxi cabs even offer free service to these students to help them get home or to the testing site. My friend Kevin’s uncle took time off work to drive Kevin to the test site, pick him up at noon to take him home, then bring him back. It is huge deal!
    (原文:slkchina.wordpress.com/



  

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