中英文翻譯--力學(xué)的基本概念

1、<p><b>　　力學(xué)的基本概念</b></p><p>　　對(duì)運(yùn)動(dòng)，時(shí)間和作用力作出科學(xué)分析的分支被稱(chēng)為力學(xué)，它由靜力學(xué)和動(dòng)力學(xué)兩部分組成。靜力學(xué)對(duì)靜止系統(tǒng)進(jìn)行分析，即在靜力學(xué)系統(tǒng)中不考慮時(shí)間這個(gè)因素，而動(dòng)力學(xué)是對(duì)隨時(shí)間變化的系統(tǒng)進(jìn)行分析。</p><p>　　通過(guò)配合表面作用力被傳送到機(jī)器的各個(gè)部件，例如從齒輪傳到軸或者是從一個(gè)齒輪通過(guò)嚙合傳遞到另一個(gè)

2、齒輪，從三角皮帶傳到皮帶輪，或者從凸輪傳到從動(dòng)件。由于很多原因，我們必須知道這些力的大小。在邊界或嚙合表面作用力的分布一定要合理，他們的大小必須在構(gòu)成配合表面材料的工作極限以?xún)?nèi)。例如，如果施加在滑動(dòng)軸承的作用力太大，那么它就會(huì)將油膜擠壓出來(lái)，并且造成金屬和金屬的接觸，使溫度過(guò)高，使滑動(dòng)軸承失效。如果作用在齒輪輪齒上的力過(guò)大，就會(huì)將油膜從齒間擠壓出來(lái)。這將會(huì)導(dǎo)致金屬表層的破裂和剝落，噪音增大，運(yùn)動(dòng)不精確，直至報(bào)廢。在力學(xué)研究中，我們主要關(guān)

3、心力的大小，方向和作用點(diǎn)。</p><p>　　當(dāng)一些物體連接在一起形成一個(gè)組合或者系統(tǒng)時(shí)，在兩個(gè)接觸的物體之間作用和反作用的力被稱(chēng)之為約束力。這些力約束各個(gè)物體使其處于特有的狀態(tài)。作用在這個(gè)物體系統(tǒng)外部的力叫做外力。</p><p>　　電力，磁力和重力是不需要直接接觸就可以施加的力的實(shí)例。不是全部但是大多數(shù)，與我們有關(guān)的力都是通過(guò)直接的實(shí)際接觸或者是機(jī)械接觸才能產(chǎn)生的。</p&g

4、t;<p>　　力是一個(gè)矢量。力的要素就是它的大小，它的方向和作用點(diǎn)，一個(gè)力的方向包括力的作用線(xiàn)的概念和它的指向。因此，沿著力的作用線(xiàn)，力的方向有正副之分。</p><p>　　沿著兩條不重合的平行線(xiàn)作用在一個(gè)物體上的兩個(gè)大小相等、方向相反的作用力不能合并成一個(gè)合力。任何作用在一個(gè)剛體上的兩個(gè)力構(gòu)成一個(gè)力偶。力偶臂就是這兩個(gè)力的作用線(xiàn)之間的垂直距離。</p><p>　　力偶

5、矩也是一個(gè)矢量，用M表示，垂直于力偶面；M的方向主要依據(jù)右手螺旋定則確定。力矩的大小是力偶臂與其中一個(gè)力的大小的乘積。</p><p>　　如果一個(gè)剛體滿(mǎn)足下列條件，那么它處于平衡狀態(tài)：</p><p>　?。?）作用在它上面的所有外力的矢量和等于零。</p><p>　?。?）作用在它上面的所有外力對(duì)于任何一個(gè)軸的力矩之和等于零。</p><p

6、>　　在數(shù)學(xué)上這兩個(gè)條件被表示為</p><p>　　所使用的術(shù)語(yǔ)“剛體”可以是整臺(tái)機(jī)器，一個(gè)機(jī)器中幾個(gè)相互連接的零件，一個(gè)單獨(dú)的零件或者是零件的一部分。隔離體簡(jiǎn)圖是一個(gè)從機(jī)器中隔離出來(lái)的物體的草圖或視圖，在圖中標(biāo)出所有作用在物體上的力和力矩。通常圖中應(yīng)該包括已知的力和力矩的大小、方向還有其他相關(guān)信息。</p><p>　　這樣得到的圖成為“隔離體簡(jiǎn)圖”，其原因是圖中的零件或物體的一

7、部分已經(jīng)從其余的機(jī)械零部件中隔離出來(lái)了，其余的機(jī)器零部件對(duì)它的作用已經(jīng)用力和力矩代替。對(duì)于一個(gè)完整的機(jī)器零部件隔離體簡(jiǎn)圖，圖上所表示出的，作用在其上面的力和力矩是通過(guò)與其相鄰或相接觸零件施加的，是外力。對(duì)于一個(gè)零件的一部分的隔離體簡(jiǎn)圖作用在切面上的力和力矩都是通過(guò)被切掉部分施加的，是內(nèi)力。</p><p>　　繪制和提交簡(jiǎn)潔、清晰的隔離體簡(jiǎn)圖是工程交流的核心。這是真實(shí)的，因?yàn)樗麄兇砹怂伎歼^(guò)程的一部分，無(wú)論這個(gè)過(guò)

8、程有沒(méi)有繪制在圖紙上，因?yàn)楹?jiǎn)圖的繪制是把思考結(jié)果進(jìn)行交流的唯一方式。無(wú)論出現(xiàn)的問(wèn)題多么簡(jiǎn)單，你都要養(yǎng)成繪制隔離體簡(jiǎn)圖的習(xí)慣。隔離體簡(jiǎn)圖的繪制加速了解決問(wèn)題的過(guò)程，大大的降低了犯錯(cuò)誤的機(jī)會(huì)。</p><p>　　使用隔離體簡(jiǎn)圖的優(yōu)點(diǎn)總結(jié)如下：</p><p>　?。?）對(duì)于一個(gè)人來(lái)說(shuō)，把詞語(yǔ)、想法和觀點(diǎn)用物理模型表示是很容易的。</p><p>　?。?）有助于幫助人

9、們觀察和理解一個(gè)問(wèn)題的各個(gè)方面。</p><p>　　（3）有助于確定解決問(wèn)題的途徑。</p><p>　?。?）有助于發(fā)現(xiàn)和數(shù)學(xué)的關(guān)系。</p><p>　?。?）他們的應(yīng)用易于記錄解題的步驟，有助于作出有關(guān)簡(jiǎn)化的假設(shè)。</p><p>　　（6）解題所用的方法可以存儲(chǔ)，供以后參考。</p><p>　?。?）他們有

10、助于你的記憶，并且易于向其他人解釋和表達(dá)你的工作。</p><p>　　在分析機(jī)器中的力時(shí)，我們幾乎總是要把機(jī)器分離成許多單個(gè)的部件來(lái)繪制標(biāo)有作用在各個(gè)部件上的力的隔離體簡(jiǎn)圖。許多部件都要通過(guò)運(yùn)動(dòng)副進(jìn)行連接。</p><p>　　在任何工程結(jié)構(gòu)中，單個(gè)的零件或部件都將受到外力，而這些力是由他們所工作的環(huán)境或條件產(chǎn)生的。如果零部件處于平衡狀態(tài)，那么外力作用的結(jié)果就是零，但是這些力共同在這個(gè)零

11、部件上施加了一個(gè)載荷，這個(gè)載荷使這個(gè)零部件有變形的趨勢(shì)，這種趨勢(shì)是內(nèi)力相互作用的結(jié)果，是在物體內(nèi)部建立起來(lái)的。</p><p>　　把載荷施加到零部件上有許多不同的方法。載荷可以被歸為如下幾類(lèi)：</p><p>　　靜載荷是一個(gè)逐漸施加的載荷，因此在一個(gè)相對(duì)很短的時(shí)間力就可以達(dá)到平衡。</p><p>　　持續(xù)載荷是一個(gè)在相當(dāng)長(zhǎng)的時(shí)間內(nèi)持續(xù)作用的載荷，例如物體的重力

12、。這種類(lèi)型的載荷被認(rèn)為是和靜載荷以同樣的方式作用著；但是，由于溫度和應(yīng)力的原因，在短時(shí)間內(nèi)加載和持續(xù)加載兩種情況下，阻力失效有所不同。</p><p>　　沖擊載荷是一個(gè)快速被施加的載荷（能源載荷）。震動(dòng)通常是由沖擊載荷引起的，直到震動(dòng)被消除才能達(dá)到平衡，震動(dòng)通常都是有阻尼力消除的。</p><p>　　重復(fù)載荷是一個(gè)被施加并且移動(dòng)過(guò)上千次的載荷。</p><p>

13、　　疲勞載荷或交變載荷的大小隨著時(shí)間而改變。</p><p>　　有人注意到上述作用在處于平衡狀態(tài)的物體上的外力和物體的內(nèi)力相互作用。因此，如果一個(gè)物體受到拉伸或是擠壓，例如在橫截面上施加一個(gè)均勻的外力，那么就會(huì)產(chǎn)生均勻的內(nèi)力，并且這個(gè)物體也會(huì)受到均勻的應(yīng)力，這個(gè)應(yīng)力被定義為</p><p>　　因此應(yīng)力是壓縮應(yīng)力還是拉伸應(yīng)力取決于載荷的性質(zhì)，它的單位是牛頓每平方米。</p>

14、<p>　　如果一個(gè)物體受到軸向載荷的作用，還產(chǎn)生力應(yīng)力，物體的長(zhǎng)度將發(fā)生變化。如果物體的原始長(zhǎng)度是，變化后長(zhǎng)度增加了，那么所產(chǎn)生的應(yīng)變?nèi)缦?lt;/p><p>　　因此應(yīng)變衡量了物體的變形程度，它是無(wú)量綱，例如它沒(méi)有單位；他是兩個(gè)具有相同單位的數(shù)量的比值。</p><p>　　因此，在載荷的作用下材料的變化實(shí)際上都是很小的，通常都用應(yīng)變來(lái)表示，其形式是應(yīng)變，當(dāng)它的形式變?yōu)闀r(shí)就是微

15、應(yīng)變。</p><p>　　拉伸應(yīng)力和應(yīng)變被認(rèn)為是正向的。拉縮應(yīng)力和應(yīng)變被認(rèn)為是負(fù)向的。因此負(fù)應(yīng)變使長(zhǎng)度減小。</p><p>　　如果材料在卸下載荷后恢復(fù)到?jīng)]加載荷是的狀態(tài)，這種材料是彈性材料。應(yīng)用于大范圍的工程材料，至少部分在負(fù)載范圍內(nèi)的彈性，其特點(diǎn)就是產(chǎn)生的變形和所施加的載荷成正比。因此載荷和它們所產(chǎn)生的應(yīng)變成比例關(guān)系，變形和應(yīng)變成比例關(guān)系，這也就意味著當(dāng)材料是彈性材料時(shí)應(yīng)力和應(yīng)變成

16、比例。因此胡克定律是</p><p>　　這則定律在一定的范圍內(nèi)適用于鐵合金材料，甚至可以以一定的精度用于其他工程材料，如混凝土，木材和有色金屬等。</p><p>　　如果材料是彈性的，當(dāng)卸下載荷時(shí)，所產(chǎn)生的變形將完全恢復(fù)；不會(huì)產(chǎn)生永久變形。</p><p>　　在材料彈性范圍內(nèi)，在胡克定律應(yīng)用范圍內(nèi)，可表示為</p><p>　　這種持續(xù)

17、的象征用來(lái)表示，被成為彈性模量或楊氏模量。因此</p><p>　　楊氏模量在拉伸和壓縮是被認(rèn)為是一樣的，對(duì)于大多數(shù)工程材料其數(shù)值都是很高的。特別是鋼，，因而應(yīng)變通常都是很小的。</p><p>　　在大多數(shù)普通工程應(yīng)用中應(yīng)變很少超過(guò)0.1%。對(duì)于任何材料，楊氏模量的精確值都是通過(guò)在材料樣品上進(jìn)行標(biāo)準(zhǔn)試驗(yàn)才能確定。</p><p>　　Basic Concepts

18、in Mechanics</p><p>　　The branch of sicientific analysis which deals with motions,time,and forces is called mechanic and is made up of two parts,statics and dynamics.Statics deals with the analysis of statio

19、nary systems,i.e.,those in which time is not a factor,and dynamics deals with systems which change with time.</p><p>　　Forces are transmitted into machine members through mating surfaces,e.g.,fron a gear to

20、a shaft or from one gear through meshing teeth to another gear,from a V belt to a pulley,or from a cam to a follower.It is necessary to know the magnitudes of these forces for a variety of reasons.The distribution of the

21、 forces at the boundaries or mating surfaces must be reasonable,and theirintensities must be within the working limits of the materials composing the surfaces.For example,if theforce operating</p><p>　　When

22、a number of bodies are connected togther to form a group or system,the forces of action and reaction between any two of the cinnecting bodies are called constraint foeces.These forces constrain the bodies to behave in a

23、specific manner.Forces external to this system of bodies are called applied forces.</p><p>　　Electic,magnetic,and gravitational forces are examples of forces that may be applied without actual physical conta

24、ct.A great many ,if not most,of the forces eith which we shall be concerned occur through direct physical or nechanical contact.</p><p>　　Force F is a vector.The characteristics of a force are its nagnitude,

25、its direction,and its point of application.The direction of a force includes the concept of a line,along which the forfe is directed,and a sense.Thus,a forceis directed positively or negatively along a line of action.<

26、;/p><p>　　Two equal and opposite foeces acting along two noncoincident parallel straifht lines in abody cannot be combined to obtain a single resultant force.Any two such forces acting on a body constitute a co

27、uple.The atm of the couple is the perpendicular distance between their lines of action,and the plane of the couple is the plane containing the two lines of action.</p><p>　　The moment of a couple is another

28、vector M directed normal to the plane of the couple;the sense of M is in accordance with the riht-hand rule for rotation.The magnitude of the moment is the product of the arm of the couple and the mafnitude of one of the

29、 forces.</p><p>　　A rigid body is in static equilibrium if:</p><p>　　(1) The vector sum of all forces acting upon it is zero.</p><p>　　(2) The sum of the miments of all the foeces a

30、cting about any single axis is zero.</p><p>　　Mathematically these two statements are expressed as</p><p>　　The term “rigid body ” as used here may be an entire machine,severral connected parts

31、of a machine,a single part,or a portion of a part.A free-body diagram is s sketch or drawing of the body,isolated from the machine,on which the forces and moments are shown magnitudes and directions as well as other pert

32、inent information.</p><p>　　The diagram so obtained is called “free” because the part or portion of the body has been freed from the remaining machine elements and their effects have been replaced by forces

33、and miments.If the free-body diagram is of an entire machine part,the forces shown on it are the external forces (applied forces) and miments exerted by adjoining or connected parts.If the diagran is a portion of a part,

34、the forces and moments acting on the cut portion are the internal forces and moments exerted by the p</p><p>　　The construction and presentation of clear and nearly drawn free-body diagrams represent the hea

35、rt of engineeting communication.This is true because they represent a part of the thinking process,whether they ate actually placed on paper or not,and because the constuction of these diagrans is the only way the result

36、s of thinking can be cimmunicated ti others.You should acquire the habit og draqong free-body diagrams no matter how simple the problem may appear to be.construction of the diagrams sp</p><p>　　The advantage

37、s of using free-body diagrams can be summarized as follows:</p><p>　　They make it easy for one to translate words and thoughts and ideas into physical models.</p><p>　　They assist in seeing and

38、understanding all facets of a problem.</p><p>　　They help in planning the attack on the problem.</p><p>　　They make mathematical relations easier to see or find.</p><p>　　Their ude

39、makes it rasy to keep track of one’s progress and helps in making simplifying assumption.</p><p>　　The methods used in the solution may be stored for future reference.</p><p>　　They assidt your

40、memory and make it easier to explain and present your work to others.</p><p>　　In analyzing the forces in machines we shall amost always need to separate the machine into its individual component and cinsteu

41、ct free-body diagrams showing the forces thet act upon each component.Many of these parts will be cinnected to each other by kinematic pairs.</p><p>　　In any engineering structure the individual components w

42、ill be subjected to external forces arising from the service conditions or environment in which the component works.If the component or member is in equilibrium,the resultant of the external forces will be zero but,never

43、theless,they together place a load on the member which tends to deform that member and which must be reacted by internal forces set up within the material.</p><p>　　There are a number of different ways in wh

44、ich load can be applied to a member.Loads may be classified with respect to time:</p><p>　　A static load is a gradually applied load for which equilibrium is reached in a relatively short time.</p>&l

45、t;p>　　A sustained load is a load that is constant over a long period od time,such as the weight of a structure.This type of load is treated in the same manner as a static load;however,for some materials and cinditions

46、 of temperature and stress,the resistance to failure may be different under short time loading and under sustained loading.</p><p>　　An impact load is a rapidly applied load (an energy load).Vibration normal

47、ly results from an impact load ,and equilibrium is not established until the vibration is eliminated,usually by natural damping forces.</p><p>　　A reprated load is a load that is applied and temoved many tho

48、usands of times.</p><p>　　A fatigue or alternating load is a load whose magnitude and sign are changed with time.</p><p>　　It has been noted above that external force applied to a body in equili

49、brum is reacted by internal forces set up within the material.If ,therefore,a bar is subjected to a uniform tension or compression,i.e. a force,which is unifoemly applied across the cross-section,then the internal forces

50、 set up ate also distributed uniformly and the bar is said to be subjected to a uniform normal stress,the stress being defined as </p><p>　　Stress may thus be compressive or tensile depending on the nature

51、of the loaad and wil be measured in units of newtons per square meter or multiples of this.</p><p>　　If a bar is subjected to an axial load, and hence a stress, the var will change in length. If the bar has

52、 an originallength L and changes in length by an amount , the strain produced is defined as follows:</p><p>　　Strain is thus a measure of the deformation of the material and is non-dimensional, i.e. it has n

53、o units; it is simply a ratio of two quantities with the same unit.</p><p>　　Since, in practice, the extensions of materials under load are very small, it is often convenient to measure the strains in the fo

54、rm of strain , i.e. microstrain,when the symbol used becomes .</p><p>　　Tensile stresses and strains are cinsidered positive in sense. Compressive stresses and strains are considered negative in sense. Thus

55、a negative strain produces a decrease in length.</p><p>　　A material is said to be elastic if it returns to its original, unloaded dimensions when load is removed. A particular form of elasticity which appli

56、es to a large range of engineering materials, at least over part of their load range, produces deformations which are proportional to the loads producing them. Since loads ate proportional to the stresses they produce an

57、d deformations are proportional to the strains, this also implies that, whilst materials are elastic, stress is proportional to str</p><p>　　This law is obeyed within certain limits by most ferrous alloys an

58、d it can even be assumed to apply to other engineering materials such as concrete, timber and non-ferrous alloys with reasonable accuracy.</p><p>　　Whilst a material is elastic the deformation produced by an

59、y load will be completely recovered when the load is removed; ther is no permanent deformation.</p><p>　　Within the elastic limits of materials, i.e. within the limits in which Hooke’s law applies, ut has be

60、en shown that</p><p>　　This constant is given the symbol E and termed the modulus of elasticity or Young’s modulus. Thus </p><p><b>　　(2.5)</b></p><p>　　Young’smodulus E

61、 is generally assumed to be the same in tension or compression and for most engineering materials has a high numerical value. Typically, for steel, so that it will be oberved from Eq.(2.5) that strains are normally very

62、 small.</p><p>　　In most common engineering applications strains rarely exceed 0.1%. The actual value of Young’s modulus for any material is normally determined by carrying out a standard test on a specimen