1、第三课 Overview of Engineering Mechanics 工程力学概述1、As we look around us we see a world full of things； Machines, devices, tools; things that we have designed, built, and used; things made of wood, metals, ceramics, and plastic. We know from experience that some things are better than others; they last lo
2、nger, cost less, are quieter, look better, or are easier to us.当我们观察我们周围就会发现世界充满“物品”：机器，设备，工具；我们已经设计，建造，并使用的物品； 木头，金属，陶瓷和塑料制品。根据我们使用的经验知道，有些物品比其它物品更好；他们使用寿命较长，费用较低，噪音更低，更好看，或者更方便我们使用。2、Ideally, however, every such item has been designed according to some set of “functional requirements” as perceived
3、 by the designers that is, it has been designed so as to answer the question, “Exactly what function should it perform?” In the world of engineering, the major function frequently is to support some type of loading due to weight, inertia, pressure, etc. From the beams in our homes to the wings of an
4、 airplane, where must be an appropriate melding of materials, dimensions, and fastening to produce structures that will perform their functions reliably for a reasonable cost over a reasonable lifetime.然而，在理想的情况下，每一件产品都是设计人员工具其对某些“功能要求”的理解而设计出来的，也就是说，在设计过程中，应该回答这样的问题，即“它应该具有哪种确切的功能？” 在工程领域，主要功能通常是承受
5、由于重力，惯性力，压力等作用的一些类型的载荷。从我们居住房屋的梁到飞机机翼，都必须有一个适当的材料，尺寸，在较合理的寿命基础上具有较合理的成本并能可靠地完成其功能的产品结构连接的组合。3、In practice, the engineering mechanics methods are used in two quite different ways:在实践中，工程力学方法常被应用在两个完全不同领域：(1) The development of any new device requires an interactive, iterative consideration of form, s
6、ize, materials, loads, durability, safety, and cost.任何新装置的研发都需要对其结构，尺寸，材料，载荷，耐久性，安全和成本的反复考虑。(2) When a device fails (unexpectedly) it is often necessary to carry out a study to pinpoint the cause of failure and to identify potential corrective measures. Our best designs often evolve through a succes
7、sive elimination of weak points.当一个装置（意外地）发生失效后，通常需要进行研究，找出失效的原因，并找出潜在的纠正措施。最好的设计往往都是不断排除薄弱环节的演变过程。4、To many engineers, both of the above processes can prove to be absolutely fascinating and enjoyable, not to mention (at times) lucrative.对许多工程师来说，上述过程既可以令人非常的陶醉又可以使人非常的愉快，更何况（有时）对我们是有利的。5、In any “rea
8、l” problem there is never sufficient good, useful information; we seldom know the actual loads and operating conditions with any precision, and the analyses are seldom exact. While our mathematics may be precise, the overall analysis is generally only approximate, and different skilled people can ob
9、tain different solutions. In the study of engineering mechanics, most of the problems will be sufficiently “idealized” to permit unique solutions, but it should be clear that the “real world” is far less idealized, and that you usually will have to perform some idealization in order to obtain a solu
10、tion.对于任何实际的问题，总是缺乏足够完整和有用的信息。我们很少准确地知道实际荷载和工作状态，因此，所做的分析工作也很少是精确的。虽然我们的数学可以准确，全面的分析一般只能近似，而且不同技术水平的人能得到不同的解。在工程力学研究领域，大多数问题要想得到唯一解就要充分的理想化，但应该清楚， “现实世界”远不非理想化程度，因此为了得到问题的解决方案不得不进行一些理想化假设。6、The technical areas we will consider are frequently called “statics” and “strength of materials”, “statics” re
11、ferring to the study of forces acting on stationary devices, and “strength of materials” referring to the effects of those forces on the structure (deformations, load limits, etc.)我们要考虑的技术领域通常被称为“静力学”和“材料力学” ， “静力学” ，指的是研究作用在固定装置上的作用力， “材料力学”指的是施加到结构的力（变形，载荷限制等）的影响。7、While a great many devices are n
12、ot, in fact, static, the methods developed here are perfectly applicable to dynamic situations if the extra loadings associated with the dynamics are taken into account, Whenever the dynamic forces are small relative to the statics loadings, the system is usually considered to be static.但是，事实上很多设备都不
13、是静态的，如果与动力学有关的额外负荷被考虑了的话，那么静力学的研究方法完全适用于动态的情况，只要动态力相对静态载荷较小，系统通常被认为是静态的。8、In engineering mechanics, we appreciate the various type of approximations that are inherent in any real problem:在工程力学中，我们非常重视与实际问题本质有关的各种类型的近似方法。Primarily, we will be discussing things which are in “equilibrium”, i. e., not a
14、ccelerating. However, if we look closely enough, everything is accelerating. We will consider many structural members to be “weightless” but they never are. We will deal with forces that act at a “point”, but all forces act over an area. We will consider some parts to be “rigid” but all bodies will
15、deform under load.首先，我们将要讨论处于“均衡”的事情，也就是说，不是加速的事情。但是，如果我们仔细观察，一切事情都是加速的。我们会认为许多结构单元是无质量的-但情况从来不是这样的。我们会处理作用在某个点上的力，但所有力都作用在某个区域上。我们将考虑某些零件是“刚性的”-但是所有机构都将在负载下变形。We will make many assumptions that clearly are false. But these assumptions should always render the problem easier, more tractable. You wil
16、l discover that the goal is to make as many simplifying assumptions as possible without seriously degrading the result.很明显，我们将做出许多虚假的假设。但这些假设总是使问题变得更容易，更容易处理。你会发现，我们的目标是使许多简化假设尽可能不严重地使结果退化。9、Generally there is no clear method to determine how completely, or how precisely, to treat a problem: If our
17、analysis is too simple, we may not get a pertinent answer; if our analysis is too detailed, we may not be able to obtain any answer. It is usually preferable to start with a relatively simple analysis and then add more detail as required to obtain a practical solution.一般没有明确的方法来确定如何完全，或者如何准确地处理一个问题：
18、如果我们的分析过于简单，我们可能无法获得相关的答案，如果我们的分析是过于详细，我们可能无法获得任何答案。它通常是最好开始用一个相对简单的分析，然后根据需要添加更多的细节获得一个实用的解决方案。10、During the past two decades, there has been a tremendous growth in the availability of computerized methods for solving problems that previously were beyond solution because the time required to solve
19、them would have been prohibitive. At the same time the cost of computer capability and use has decreased by orders of magnitude（数量级）. We are experiencing an influx of “personal computers” on campus, in the home, and in business.在过去的二十年中，在解决问题的计算方法的有效性方面得到突飞猛进的增长。这些方法以前超越了解解决方案，因为时间的约束，解决这些问题以前是望而却步。同时，计算机的能力和使用成本下降了几个数量级。我们正在经历一个“个人电脑”在校园内，在家里，并在商业中大量涌现的时代。
