Lesson2ForcesandTheir.doc

专业英语Lesson 2 Forces and TheirEffectsTerminologyText tourTerminologyCompressiveServing to or able to compress压缩的：起压缩作用或能够压缩的TurningThe shaping of metal or wood on a lathe车削：用车床将金属或木材加工成型的过程Lathe 车床 Rectilinear,rektiliniMoving in, consisting of, bounded by, or characterized by a straight line or lines:直线的：以直线运动的，向直线组成的，以直线为边的，具有直线之特点的 micrometer 千分尺ThreadA helical or spiral ridge on a screw, nut, or bolt螺纹：螺钉、螺母或螺栓上螺旋型的凸棱ShaperText tourA study of any machine or mechanism shows that each is made up of movable parts. These parts transform a given motion to a desired motion. In other words, these machines perform work. Work is done when motion results from the application of force. Thus, a study of mechanics and machines deals with forces and the effects of forces on bodies.参考译文：研究表明机器或者机构都是由若干运动零件组成的。这些零件将给定运动转换成目标运动。换句话说，这些机器做功。因为力的作用产生运动而做功。因此，力学和机械（学）研究力以及力对物体的影响。A force is a push or pull. The effect of a force either changes the shape or motion of a body or prevents other forces from making such changes. Every force produces a stress in the part on which it is applied. Forces may be produced by an individual using muscular action or by machines with mechanical motion.参考译文：力分为推力和拉力，力可以改变物体的形状或运动状态或者防止其他力作这样的改变。施加在零件上的力会在其内部产生应力。力可以由个人的肌肉活动或者由机器的机械运动产生。Forces are produced by physical or chemical change, gravity, or changes in motion. When a force is applied which tends to stretch an object, it is called a tensile force. A part experiencing a tensile force is said to be intension. A force can also be applied which tends to shorten or squeeze the object. Such a force is a compressive force.参考译文：力由物理或化学变化、地心引力或者运动变化产生。当力用于拉伸物体时叫拉力。零件受拉力作用被称为受拉（处于拉伸状态）。力也可用于缩短或挤压物体，这种力叫压力。A third force is known as a torsional force, or a torque since it tends to twist an object. Still another kind of force, which seems to make the layers or molecules mlikjuls of a material slide or slip on one another, is a shearing force.参考译文： （我们知道的）第三种力是扭转力，或者叫扭矩，因为它会扭曲物体。还有另一种力，它似乎会使材料的各层或分子间产生相对滑移，称为剪切力。Each of these forces may act independently or in combination. For example, a downward force applied on a vertical steel beam tends to compress the beam. If this beam is placed in a horizontal position and a load is applied in the middle, the bottom of the beam tends to stretch and is in tension. At the same time, the top area is being pushed together in compression. If the compressive and tensile forces are greater enough to make the layers of the material slide upon each other, a shearing force results.参考译文：这些力都可以单独或者组合起来起作用。例如，一个向下的力施加在垂直钢梁上会压缩它。如果将梁水平放置，载荷施加在梁的中央，则梁的下方会伸长而受拉。同时梁的上方被压缩而受压。如果压力和拉力足够大到使材料的各层产生相对滑移，则会出现剪切力。The turning of a part in a lathe is another example of several forces in action (as shown in Figure 1).1Fig. 1 The turning of a part in a lathe1. turning of a part 这里指“对一个零件进行车削加工”。全句译为：几个力同时作用的另外的一个例子是在车床上对零件进行车削加工（如图1所示）。As the work revolves and the cutting tool moves into the work, the wedgingaction of the cutting edge produces a shear force. This force causes the metal to seem to flow off the work in the form of chips.22. chip 这里指“切屑”。work 这里指“工件，即 workpiece ”。全句可译为：工件旋转时刀具切入工件，切削刃的楔入动作产生剪切力。这个力使得金属看起来像以切屑的形式从工件上流出来一样。If this workpiece is held between the centers of the lathe, the centers exert a compressive force against the work.33. centers of the lathe 意为“车床的两个顶尖”。全句可译为：如果一个工件被安装在车床的两个顶尖The lathe dog which drives the work tends to produce a shearing force. The pressure of the cutting tool against the work produces tension and compression, as well as a shearing action.参考译文：驱动工件的车床夹头会产生一个扭转力。刀具顶在工件上的压力会同时产生拉力和压力，以及剪切作用。Considerable attention is given to the action of centrifugal force in grinding wheels. That is, the bonding agent that holds the abrasive particles on the wheel must be stronger than the forces which tend to make the revolving wheel fly apart at high speeds. For this reason, the speed of a grinding wheel should not exceed the safe surface speed limit specified by the manufacturer. Centrifugal force increases with speed.参考译文：对磨削用的砂轮来讲离心力的影响要特别给以关注。即把磨料颗粒粘合在砂轮上的粘合剂它的粘合力必须大于使高速旋转的砂轮分崩离析的离心力。因此，砂轮的转速不能超过制造商给定的安全表面速度限制。离心力随转速的增加而增大。The principles of centrifugal force are used in the design of centrifuge-type machines. Some centrifuges are used to separate chemicals; others are used to remove impurities in metals by centrifugal casting processes.Centrifugal force principles are also used in common appliances such as clothes dryers and in devices to control motor speeds and accelerate moving machines.参考译文：离心力的这种原理用在离心型机器的设计中。一些离心分离机用于分离化学制品；还有的通过离心铸造工艺分离金属中的杂质。离心力作用也有普通的应用如干衣机、控制发动机转速的装置以及用于加速运动的机器。Centripetal force causes an object to travel in circular path. This action is caused by the continuous application of forces which tend to pull the object to the center. In other words, the inward force which resists the centrifugal force is called the centripetal force. The centripetal force of objects spinning at a constant rate produces an acceleration toward the center which is equal and opposite to the centrifugal force.参考译文：向心力使物体沿着圆形轨道运动。这种现象的产生是由于将物体拉（轨道）中心的力的持续作用。换句话说，这种抵抗离心力的向内的力就叫向心力。以固定速率作旋转运动的物体的向心力产生一个向心的加速度，这个向心力与离心力大小相等，方向相反。The materials used in the construction of rapidly moving machine parts and mechanisms must be structurally strong enough to provide the centripetal force required to hold the parts to a circular path. At the same time, the materials must be able to withstand the centrifugal force which tends to pull the parts apart.参考译文：用于制造高速运动的机械零件和机构的材料组织上必须有足够强度，以提供保持零件在圆形轨道上所需的向心力。同时，这种材料必须能经得起可将零件拉裂的离心力的作用。Motion and the basic laws which affect motion are important considerations because of the numerous applications of these principles to produce work through mechanical devices.参考译文：对运动和影响运动的基本定律需要重视，因为有许多根据这些原理通过机械装置做功的应用。There are two primary mechanical motions: rotary and rectilinear. These terms suggest that rotary motion is a circular movement around a center line and rectilinear motion is a straight line motion.参考译文：有两个主要的机械运动：回转运动和直线运动。这些术语表明（顾名思义），回转运动是一种绕着一根中心线的环状运动，而直线运动是沿着一根直线的运动。For either rotary or rectilinear motion, it is possible, with added mechanical devices, to produce other forms of motion such as intermittent motion and reciprocating motion.参考译文：对回转运动或直线运动来说，如果可能的话，加上机械装置则可产生其他形式的运动如间歇运动和往复运动。Rotary Motion. The motion that is commonly transmitted is rotary motion. This type of motion may be produced with hand tools or power tools. Rotary motion is required to drill holes, turn parts in a lathe, mill surfaces, or drive a generator or fan belt.44. rotary motion 意为“回转运动”。全句可译为：回转运动 通常称为回转运动，这种运动可以由手动工具或动力工具产生。钻孔、在车床上车削零件、铣平面、驱动发电机或风扇的带等都需要回转运动。Rectilinear ,rektilini Motion.The feed of a tool on a lathe, the cutting of steel on a power saw, or the shaping of materials are all situations in which rectilinear or straight line motion produces work. In each of these situations a part or mechanism is used to change rotary motion to straight line motion. The screw of a micrometer and the threads in a nut are still other applications where the direction of motion is changed from rotary to rectilinear.55. the screw of a micrometer 意为“千分尺中的螺杆”全段可译为：直线运动 车床上刀具的进给、在锯床上锯钢材以及材料的刨削等都是直线运动做功的情况。每个这些（类）情况都要用一个零件或机构将回转运动转化为直线运动。千分尺中的螺杆和螺母中的螺纹是把运动方向从转动变为直线的另外一些应用实例。Harmonic and IntermittentMotion.6 Any simple vibration,such as the regular back-and-forth movement of the end of a pendulum, is simple harmonic motion.76. harmonic and intermittentmotion 意为“谐波运动和间歇运动”。7. simple harmonic motion 意为“简谐运动”。全句可译为：任何简单的振动，例如摆的下端有规律的往复运动是简谐运动。However, many manufacturing processes require intermittent or irregular motion. For example, the fast return stroke of a power hacksaw or shaper ram is desirable because no cutting is done on the return stroke. Therefore, as more time is saved in returning the cutting tool to the working position, the less expensive is the operation.参考译文：然而，很多制造工艺要求间歇运动或不规则运动。例如，电动弓锯或刨床滑枕的快速回程（快回）就是令人想要的结果，因为在回程中没有进行切削。因此，在刀具回到工作位置的过程中节省的时间越多，这道工序的成本越低。The combinations of rotary and rectilinear motion obtainable are unlimited because of the large variety of parts such as gears, cams,pulleys, screws, links, and belts which can be combined in many arrangements.参考译文：可得到的回转运动和直线运动的组合是无限的，因为有大量的零件如齿轮、凸轮、带轮、螺杆、连杆和带等可以以不同的排列形式组合起来。Lesson 3 Overview ofEngineering MechanicsTerminologyText tourTerminologyInteractiveActing or capable of acting on each other互相作用的或能互相作用的Iterative itrtivCharacterized by or involving repetition, recurrence, reiteration, or repetitiousness ,repitisnis重复的, 反复的, 迭代的：以反复、循环、重述或重复为特征的，或与之相关的Pinpoint 精确地定位或确认To locate or identify with precision精确地定位或确认Equilibrium ,i:kwilibrimA condition in which all acting influences are canceled by others, resulting in a stable, balanced, or unchanging system平衡，均衡：一种所有动作的影响都互相抵消，整个系统处于平稳的、均势的、不变的状态。Tractable Easily handled or worked易于处理或操作的Order of magnitude ：A range of values between a designated lower value and an upper value ten times as large, such as the masses of Earth and the sun differ by five orders of magnitude数量级：处于相差10倍的两个数值之间的那一段数值的范围。例如，地球与太阳的质量相差五个数量级。Text tourAs 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 plastics. We know from experience that some things are better than others; they last longer, cost less, are quieter, look better, orare easier to use.参考译文：看看我们周围可以发现一个充满了“东西”的世界：机器、装置、工具等等；这些东西是我们设计、建造和使用的；这些东西是由木材、金属、陶瓷和塑料制成的。我们从经验知道一些东西比其他的要好；它们寿命更长，成本更低，更安静，更好看，或更易于使用。Ideally, however, every such item has been designed according to some set of “functional requirements” as perceived by the designersthat is, it has been designed so as to answer the question, “Exactly what function should it perform?”11. so as to 意为“使得，如此，以至于”。全句可译为：然而，在理想的情况下，每一件产品都是设计人员根据其对某些“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“它应该具有那种确切的功能？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 airplane, there must be an appropriate melding of materials, dimensions, and fastenings to produce structures that will perform their functions reliably for a reasonable cost over a reasonable lifetime.参考译文：在工程世界里，主要的功能常常是去支撑一些由重量、惯性、压力等产生的负荷。 从我们家里的房梁到飞机的机翼，一定有适当的材料、尺寸、紧固件的组合应用，以制造出在合理的生命周期里以合理的成本满足功能要求的结构。In practice, the engineering mechanics methods are used in two quite different ways:The development of any new device requires an interactive, iterative consideration of form, size, materials, loads, durability,safety, and cost.参考译文：在实践中，工程力学方法有两个很不相同的用途：（1）任何一个新装置的设计都需要在形式、尺寸、材料、载荷、耐久度、安全性和造价等方面进行反复的、交互式的考虑。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.2 Our best designs often evolve through a successive elimination of weak points.2. carry out 意为“进行，完成”， correct measure 意为“改正措施”。 全句可译为：当一个装置意外地失效后，通常需要进行研究工作，来找出失效的原 因和确定可能的改正措施。我们最好的设计通常是通过逐渐的消除缺点来实现的。To many engineers, both of the above processes can prove to be absolutely fascinating and enjoyable, not to mention (at times) lucrative.参考译文：对许多工程师来说，以上两个过程能带来迷人的、令人愉悦的体验，更不用说可以获利了。In any “real” 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.3 While our mathematics may be precise, the overall analysis is generally only approximate, and different skilled people can obtain different solutions.3. operating condition 意为“工作状态，工作条件”。全段可译为：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。虽然数学可能是精确的，但是全面分析通常只是近似的，而且不同的专业人员可得到不同的（分析）结果。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 solution.参考译文：在工程力学研究中，大部分问题要足够理想化以获得唯一解，但我们要清楚 “真实世界”远不是理想的，以致我们常常不得不做一些理想化（简化）以获得一个解。The technical areas we will consider are frequently called statics and strength of materials, statics referring 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.).参考译文：我们经常研究的技术领域叫“静力学”和“材料力学”，“静力学”研究作用于静态装置上的力，而“材料力学”是指这些力在结构上的影响（变形、负荷极限等）。While a great many devices are not, 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 static loadings, the system is usually considered to be static.参考译文：然而很多装置实际上不是静态的，如果与动力学有关的额外负荷已经有考虑了，那么这里的方法用于动态环境也是完美的。无论何时（只要）动态力相对静态负荷要小，系统常常被认为是静态的。In engineering mechanics, we appreciate the various types of approximations that are inherent in any real problem:4be inherent in 意为“为所固有，是的固有性质”。全句可译为：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 accelerating. 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 deform under load.参考译文：首先，我们要讨论的是处于“平衡”状态即没有加速的物体。然而任何东西都在加速，如果我们观察足够仔细的话。我们认为很多结构件是“没有重量”的但它们从来不是这样；我们认为力作用在一个“点”上，但所有力都作用在一个区域上；我们认为有些零件是“刚性的”但所有物体在负载作用下都会变形。We will make many assumptions that clearly are false. But these assumptions should always render the problem easier, more tractable. You will discover that the goal is to make as many simplifying assumptions as possible without seriously degrading the result.参考译文：我们会作一些明显是错误的假设。（但）这些假设常可使问题简化，更容易驾驭。你会发现目标是只要不使结果严重降级（歪曲），就尽可能多的做出简化的假设。Generally there is no clear method to determine how completely, or how precisely, to treat a problem: If our 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.参考译文：通常没有明确的方法去确定怎么完整（全面）或精确地去处理问题：如果我们的分析太简单，（我们）可能不会得到一个中肯的答案；如果我们的分析太详细，（我们）也许得不到任何答案。通常更好的是从相对简单的分析开始，添加一些需要的细节以获得实用解。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 them would have been prohibitive. At the same time the cost of computer capability and use has decrea