第 1 页 共 11 页 Ch 2 Overview of Engineering Mechanics
As we look around us we see a world full of ‘things’: machines, devices, tools; things that we have designed,
当我们环顾四周，我们可以看到充满物质的世界：机器，仪器，工具：这些被我设计、建造和使用过
built and used; things made of wood, metals, ceramics, and plastics. We know from experience that some
的东西： 这些东西是由木头，金属，陶瓷和塑料做成。我们从经验可以知道一些东西比别的东
things are better than others; they last longer, cost less, are quieter, look better, or are easier to use.
比别的东西好： 他们比较耐用，低成本，安静，看起来好点，或者使用比较简单。
Ideally, however, every such item has been designed according to some set of ‘functional requirements; as
不管怎么样，最理想的是这样一个东西都是被设计者所认为的那样根据某些“使用要求”设计出来的。
perceived by the designers—that is, it has been designed so as to answer the question, ‘Exactly what function
也就是说他的设计是为了回答这些问题，确切的说它应该执行怎么样的功
should it perform?’ In the world of engineering, the major function frequently is to support some type of
能？ 在工程的领域里，最主要的功能是经常支持一些由于重力，惯性，压力等作用产
loading due to weight, inertia, pressure etc. From the beams in our homes to the wings of an airplane, there
生的荷载。 从我们家里的梁柱要飞机的双翼，这些都是由材料尺寸和
must be an appropriate melding of materials, dimensions, and fastenings to produce structures that will
紧固件适当的组合，从而生产的结构在合理成本和使用周期的基础上可靠的履行它的职责。
perform their functions reliably for a reasonably cost over a reasonable lifetime.
In practice, engineering mechanics methods are used in two quite different ways
:在实践中，工程力学方法用于两种完全不同的方式。
1. The development of any new device requires an interactive, iterative consideration of form, size, materials,
任何新设计的发展要求都在尺寸，材料，荷载，耐久度，安全性和成本等方面进行交互、反复的考虑。
loads, durability, safety, and cost.。
2. When a device fails (unexpectedly) it is often necessary to carry out a study to pinpoint the cause of failure
当一个设计以外的失败的时候，必须经常的展开研究去查明失败的原因和确定可能的校正措施。
and to identify potential corrective measures. Our best designs often evolve through a successive elimination of
我们最好的设计是经常通过逐渐的消除缺点来实现的。
weak points.
To many engineers, both of the above processes can prove to be absolutely fascinating and enjoyable, not to
对于许多工程师，上述的过程被证明它是极具吸引力和令人愉悦的，更不用说（有时）是有利可图的。
mention (at times) lucrative.
第 2 页 共 11 页 In any ‘real’ problem there is never sufficient good, useful information; we seldom know the actual loads and
在任何真正的问题当中从未有足够好的，有用的信息，我们很少知道实际的荷载和任何精确的工作条
operating conditions with any precision, and the analyses are seldom exact. While our mathematics may
件。 并且分析的结构很少是精确的。虽然我们的数学可以很精确，
precise, the overall analysis is generally only approximate, and different skilled people can obtain different
所有的分析结果一般仅仅是近似的，和不同掌握技术的人可以获得不同的解决方案。
solutions. In the study of engineering mechanics most of the problems will be sufficiently ‘idealized’ to
在工程力学的研究当中，大多数的问题都将被足够的理想化，以便允许得到唯一的答案。
permit unique solutions, but it should be clear that the ‘real world’ is far less idealized, and that you usually
但是我们应该清楚意识到“真实的世界”远不是理想化的，你通常不得不执行
will have to perform some idealization in order to obtain a solution.
一些理想化以便获得答案。
The technical areas we will consider are frequently called ‘statics’ and ‘strength of materials’, ‘statics’
我们将考虑的技术领域通常叫做“静力学”和“材料力学”。静力学是指力作用于静止物体上的一种
referring to the study of forces acting on stationary devices, and ‘strength of materials’ referring to the effects
研究， “材料力学”是指那些力作用在结构上的
of those forces on the structure (deformation, loading limits, etc.).
影响（变形，荷载极限等）。
While a great many devices are not, in fact, static, the methods developed here are perfectly applicable to
然而在实际上许多的设备并不是静力学的问题，如果额外的荷载被考虑，这里的研究方法完全适用于
dynamic situations if the extra loadings associated with the dynamics are taken into account (we shall briefly
动力学情形，以至于在动力的情况下也被考虑在内（我们应该简单的提一下是怎么样被做到的）
mention how this is done). Whenever the dynamic forces are small relative to the static loadings, the system
只要动力学的力相对于静力荷载来说很小，这个系统通常可以考虑为静力学
is usually considered to be static.
的问题。
In engineering mechanics, we will begin to appreciate the various types of approximations that are inherent
在工程力学里，我们将开始正确评价各种类型在实际问题中本身就存在的近似方法。
in any real problem:
Primarily, we will be discussing things which are in ‘equilibrium’, i.e., not accelerating. However, if we look
首先，我们将会讨论处于平衡及不产生加速度的物体。然而如果我们看的够仔细，所有东西是运动的。
closely enough, everything is accelerating. 第 3 页 共 11 页 We will consider many structural members to ‘weightless’ but they never are.
我们会把一些结构构件考虑成失重，但他们从来都不是。
We will deal with forces that act at a ‘point’—but all forces act over an area.
我们要处理的力作用在一个点上——但是所有的力作用在一个区域上。
We will consider some parts to be ‘rigid’—but all bodies will deform under load.
我们会认为一些结构是刚性的——但是所有的构件都在何在下变形。
We will make many assumptions that clearly are false. But these assumptions should always render the
我们将做很多显然错误的假设。 但是这些假设应该总是使得问题更加容易处理。
problem easier, more tractable. You will discover that the goal is to made as many simplifying assumptions
你将发觉目标是尽可能的做简单的假设，而没有严重降低结果。
as possible without seriously degrading the result.
Generally there is no clear method to determine how completely, or how precisely, to treat a problem. If our
通常没有明确的方法去可以完美的确定或者精确的处理这个问题。 如果我
analysis is too simple, we may not get a pertinent answer; if our analysis is too detailed, we may not be able
们的分析结构太过于简单，我们可能得不到正确的答案，如果我们的分析太过于详细，我们可能不能
to obtain any answer. It is usually preferable to start with a relatively simple analysis and then add more
或者任何的答案。 通常我们更喜欢的是从相对简单的分析开始，然后增加更多的细节以便于得到
detail as required to obtain a practical solution.
必须的实际结果。
During the past two decades, there has been a tremendous growth in the availability of computerized
在过去二十年中，在计算机解决问题方法的实用性有了相当大的发展，
methods for solving problems that previously were beyond solution because the tie required to solve them
这些问题以前是不能解决的，因为需要解决他们时间不允许。
would have been prohibitive. At the same time the cost of computer capability and use has decreased by
同时计算机的内存能力和使用成本已经降下了许多个数量级。
orders of magnitude. We are experiencing an influx of ‘personal computers; on campus, in the home, and in
我们正在经历个人计算机进入校园，家庭和商场的时代。
business.
