机电英语(英汉对照)力学里的基本概念.doc

Lesson 4
Basic Concepts in Mechanics
Section 1 Text

Introduction
In its original sense，mechanics refers to the study of the behavior of systems under action of forces. Statics deals with cases where the forces either produce no motion or the motion is not of interest.Dynamics deals properly with motions under forces. Mechanics is subdivided according to the types of systems and phenomena involved.
An important distinction is based on the size of the system Those systems are large enough and can be adequately described by the Newtonian law of classical mechanics.In this category，for example，are celestial mechanics the study of the motions of planets，stars . and other heavenly bodies and fluid mechanics which treats liquids and gases on a macroscopic scale. Fluid mechanic is a part of a larger field called continuum mechanics , involving any essentially
continuous distribution of matter whether rigid，elastic，plastic，or fluid. On the other hand zhe behavior of microscopic systems such as molecules，atoms , and nuclei can be interpreted only by
the concepts and mathematical methods of quantum mechanics.From
its inception,quantum mechanics had two apparently different mathematical froms：zhe wave mechanics of E。Schrodinger，which emphasizes the spatial probability distributions in the quantum states，and zhe matrix mechanics of W.Heisenberg，which emphasizes the transitions between states。These are now known to be equivalent.
Mechanics may also be classified as nonrelativistic and relativistic mechanics，the latter applying to systems with material velocities comparable to the velocity of light. This distinction
pertains to both classical and quantum mechanics.
Finally，statistical mechanics uses the methods of statistics for both classical and quantum
systems containing very large number of similar subsystems to obtain their large-scale properties.

Basic Concepts in Mechanics
That branch of scientific analysis which deals with motions, times，and forces is called mechanics and is made up of two parts:statics and dynamics.Statics deals with the anslysis of stationary systems e._those in which time is not a factor，and dynamics deals with systems which change with time.
Forces are transmitted into machine members mating surfaces，e. g.，from a gear to a shaft or from one gear through meshing teeth to another gear，from a rod through a bearing to a lever .from a V-belt to a pulley，or from a carn to a follower。it is necessary to konow zhe magnitudes of these forces for a variety of reasons. The distrihution of the forces at zhe boundaries or mating surfaces must be reasonable and their intensities must be within zhe working limits of the materials composing the surfaces. For example，if the force operating on a sleeve bearing becomes too high, it will squeeze out the oil film and cause metal contact，overheating，
and rapid failure of the bearing. If the forces between gear teeth are too large， zhe oil fim may be
squeezed out from between them. This could result in flaking and spalling of the mdGal,noise，rbugh motion，and eventual failure. In the study of dynamics we are principally interested in determining the magnitude，direction .and location of the forces。
Some of the terms used in this phase of our studies are c}fined below.
Force. Our earliest ideas concerning arose because of our desne to push ，lift，or pull various o功ects.so force is the action of one body acting on another. Our intuitive concept of force includes such ideas as p lace of application，direction and magnitude，and these are called the characteristics of a force.
Matter. Matter is any material or substance; if it is completely enclosed it is called a body.

. Mass。 defined mass as the quantity of a body as measured饰its volume and
This is not a very satisfactory definition because density is the mass of a unit vo We can
Excuse Newton by surmising that he perhaps did not mean it to be a definition. Nevertheless, he
recognized the fact that all bodies possess some inherent property that is different from weight. Thus .a moon rock has a certain constant amount of substance，even though its moon weight is
different from its earth weight. This constant of substance or quantity of matter，is called the
mass of the rock.
Inertia. Inertia is the property of mass that causes it to resist any effort to change its motion.
Weight. Weight is the of gravity acting upon a The following pertinent
The great advantage of SI units is that there is one , and only one unit for each physical
meter for length，the kilogram for mass,the Newton for force.the second for time，etc. To consistent with this unique feature，it follows that a given unit or word should not be used as an accepted technical name for two physical quantities. However, for generations the term "weight" Gas been used in both technical and non-technical fields to mean either the force of gravity acting on a body or the mass of a body itself.
Particle. A particle is a body whose dimensions are so small that they may be neglected:
Rigid Body。All bodies are either elastic or plastic and will he deformed if acted upon by forces. When the deformation of such bodies is small they are frequently assumed to be
rigid , i ，e.，incapable of deformation，in order to simplify the analysis.
Deformation Body. The rigid-body assumption can not be used when internal stresses and strains due to the applied forces are to be analyzed Thus we consider the body to be capable of deforming. Such analysis is frequently called elastic-body analysis，using the additional assumption that the body remains elastic within the range of the applied
Newton's Laws. Newton's three laws are：
Law l:If all the forces acting on a particle are balanced，the particle will either remain at rest or will continue to move in a straight line at a uniform velocity.
Law 2:If the forces acting on a particle are not balanced， the particle will experience an acceleration proportional to the resultant force and in the direction of the resultant [4]
Law 3:When two particles react, a pair of interacting comes into existence . These have the same magnitudes and opposite senses， and they act along the straight line common to the two particles.
Mechanics deals with two kinds of quantities-scalars and vectors. Scalar quantities are those with which a magnitude alone is associated. Examples of scalar quantities in mechanic‘ time volume，density，speed , and mass. Vector quantities, on the other hand, direction as well as magnitude. Examples of vectors are displacement velocity , acceleration force moment, and momentum.
第四课力学里的基本概念

引言
从本义上来说，力学所研究的是系统在受力情况下的运动情况。静力学研究的是，在力
的作用下，系统没有发生运动或者运动的程度很小可以忽略不计。同样，动力学研究的是系
统在力的作用下发生了相应的运动情况。根据系统的类型和所涉及的现象，力学又可以细分
为很多学科。
系统的大小是一个非常重要的分类依据，这系统都非常大，而且完全符合牛顿经典力
学。在这种分类方式里，比如天体力学，研究的是行星、恒星以及其他天体的运动，而流体
力学研究的对象是宏观意义上的液体和气体的运动。流体力学是连续介质力学或者某些物理
学家描述的经典控制理论里的一个组成部分，涉及物质的本质组成，是否具有刚性、弹性、
塑性或者流动性。另外，微观系统比如分子、原子和核子的运动情况只能通过量子力学的数
学统计方法来描述。一开始，量子力学被分为两个截然不同的数学形式:薛定愕波动力学，
重点是研究空间里量子的可能分布;海森堡的矩阵力学，主要是研究物质间的相互转变。现
在看来，这两种力学其实是等价的。
力学也可以分为非相对论力学和相对论力学
，后者研究是这样的系统，其中物质的速度可与光速相比。这种特性同时符合经典力学和量子力学的定义。
最后，统计力学是利用统计的方法来分析经典力学及量子力学系统中含有大量相似子系统的宏观特征。

力学里的基本概念
科学分析研究运动、时间和力关系的学科被称为力学，它由两部分组成:静力学和动力
学。静力学研究的是静止状态下的物体受力情况，因此时间不是一个变量，而动力学研究的
是物体在受力情况下，其随时间变化所产生的运动。
力通过机器零件间的配合面进行传递，比如从一齿轮传递到轴，或者通过轮齿的啮合从一个齿轮传递到另一个齿轮，或者通过轴承从一个轴传递到另一个轴，从一个V带传递到带轮，从主动轮传递到从动轮等。所以，我们要知道的就是这各种各样的力的大小情况。在啮合处或接合面处的力的分布必须是合理的，它们的强度大小必须在接合面处的材料可以滚受的范围之内。比如，施加在轴承上的力如果过大，它将在接合面处挤压出油膜，并产生机械碰撞、过热甚至轴承的运动失效。如果齿牙间的力过大，油膜可能还从它们之间挤压出来，这将导致压碎、轮齿的断裂、噪声以及振动，并最终导致运动失效。在动力学中，我们主要研究的就是作用力的大小、方向以及作用位置。
力学中涉及的一些参数和概念如下文所述:
作用力我们最初认为力是在我们推、举或拉各种各样的物体时产生的，因此，力是一
个物体作用于另一个物体的行为。我们对力的直观的认识，是它的大小、方向和作用点，这
三个量也被称为力的三要素。物质。物质是任何材料的基本属性，如果它是完全封闭的，我们就称之为物体。
物质物质是任何材料的基本属性，如果它是完全封闭的，我们就就之为物体。
质量,牛顿对质量的定义是用体积和密度测量的物体的数盘，这不是一个理想的概念，
因为密度是单位体积上的质量。。我们可以理解牛顿的表述，他可能并不想把这个作为质量的定义。不过，他认识到了任何物体都具有「一个内在的不同于重量的属性，比如一个月球陨石具有固定的物质组成，虽然它在月球上和地球上的重量不一样。它所包含的这个固定不变的物质的数量就称为质量。
惯性。惯性是质量抵抗任何使其发生运动的属性。
重量,重量指的是重力作用于物质的力的大小，下面的定义是比较合理的，
采用标准单位的最大好处就是对于每一个物体都有一个唯一的量纲，如长度单位用米、质量单位为千克、力用牛顿、时间单位为秒等。按照与标准单位对应的原则，任何一个物体
的量纲都不能有两种专业名字。但是，长期以来，人们习惯用重量来表示重力和质量的值。
粒子:粒子的体积很小，可以忽略不计。
刚体:一切物体要么是弹性体，要么是塑性体。它们在受力作用时会产生变形。如果一个物体变形量很小，为了简化分析，称其为刚体，也就是说，刚体不产生变形
变形体:当需要对受外力作用产生的内部应力和应变进行分析时，就不能使用刚体的假
设。通常称这种分析为弹性体分析，并且使用另外的假设，即物体在一定范围内的外力作用
下保持弹性。

牛顿定律

定律1:如果作用在物体上所有的力是平衡的，物体将保持静止或做匀速直线运动。
定律2:如果作用在物体上的力是不平衡的，物体将沿着合外力的方向做加速运动。
定律3:两个物体间的作用力和反作用力总是大小相等、方向相反，并且作用在同一直
线上。
力有两个量的表达形式:标量和矢量。标量只考虑力的大小，比如力学里的时间、体
积、密度、能量、质量等。而矢量既表征力的大小，还表征力的方向，比如位移、速度、加
速度、力、动量等。