Lesson 2 Basic Concepts in Mechanics

Lesson 2 Basic Concepts in Mechanics

Words and Expressons

Terminology术语,专有名词

[,t?:mi'n?l?d?i]

Mechanics [m?'k?n?ks] 力学（用作单数）；机械学（用作单数）

Mechanical 力学的，机械的

Structural mechanics[力]结构力学; 布局力学

fluid mechanics流体力学，液体力学

statics （名）静力学，静止状态static（形）静止的，静态的

dynamics （名）动力学，动力特性dynimic （形）动态的

static force静力; 静力电流

static pressure静压

static analysis静态分析

static characteristics 静态特性

gravitational [,ɡr?v?'te???n?l]adj. [力] 重力的，[力] 引力的

gravity （名）重力，地心引力

gravitational acceleration重力加速度

gravitational force万有引力，地心引力

meshing n. [机] 啮合；网格划分

meshing gears啮合齿轮

bearing n. [机] 轴承，支撑

bearing capacity 承载力

rolling bearing滚动轴承

ball bearing滚珠，钢球；滚珠轴承

cam [k?m] n. 凸轮

cam mechanism凸轮机构

cam drive凸轮传动，凸轮驱动

journal ['d???n(?)l] n. 日报，杂志，【机械学】轴颈

journal bearing径向轴承

core journal核心期刊

academic journal学报，学术期刊

parallel ['p?r?lel] n. 平行线；对比vt. 使…与…平行adj. 平行的；类似的，

相同的

Parallel communication并行通信; 并行; 并行通讯; 平行传输Parallel algorithm[计]平行演算法; [计]并行算法

i n parallel并行的；[电]并联的；平行的

parallel with平行；与…比较

parallel connection并联

resultant [r?'z?lt(?)nt]n. 合力；结果；[化学] 生成物adj. 结果的；合成的resultant force[力]合力; 总反力; 协力; 净力

stress resultant[力]应力合力

inertia[?'n????] n. [力] 惯性；惰性，迟钝；不活动

inertia force[力]惯性力

moment of inertia惯性矩

Inertia effect惯性效应; 惯性效果; 惯性作用

deformation [,di?f??'me??(?)n]n. 变形

plastic deformation [化]塑性变形，塑性应变

elastic deformation弹性形变

thermal ['θ??m(?)l] deformation热变形；热形变

acceleration [?ksel?'re??(?)n] n. 加速，促进；[物] 加速度

angular acceleration角加速度

constant acceleration等加速度

maximum ['m?ks?m?m] acceleration最大加速度

scalar ['ske?l?] 数量（的），标量（的）

vecto r ['vekt?] n. 矢量；带菌者；航线

vector space n. [数]向量空间

normal vector法向量；法向向量；法向矢量

vector graphics ['gr?f?ks] 向量图形；向量图

momentum [m?'ment?m] n. 势头；[物] 动量；动力；冲力

momentum equation动量方程

momentum conservation[?k?ns??ve??n]动量守恒

momentum transfer动量传递；动量交换

Text：

The branch of scientific analysis which deals with motions, time, and forces is called mechanics and is made up of two parts, statics and dynamics. Statics deals with the analysis of stationary systems, i.e., those in which time is not a factor, and dynamics deals with systems which change with time.

对运动、时间和作用力进行科学分析的分支称为力学。它由静力学和动力学两部分组成。静力学对静止系统进行分析，即在其中不考虑时间这个因素，动力学对随时间而变化的系统进行分析。

When a number of bodies are connected together to form a group or system, the forces of action and reaction between any two of the connecting bodies are called constraint forces. These forces constrain the bodies to behave in a specific manner. Forces external to this system of bodies are called applied forces.

当一些物体连接在一起形成一个组合体或者系统时，任何两个相互连接的物体之间的作用力和反作用力称为约束力。这些力约束着各个物体，使其处于特定的状态。从外部施加到这个物体系统的力称为外力。

Electric, magnetic, and gravitational forces are examples of forces that may be applied without actual physical contact. A great many, if not most, of the forces with which we shall be concerned occur through direct physical or mechanical contact.

电力、磁力和重力是可以不需要真正的实际接触而施加力的例子。即便不是大部分，仍有许多与我们有关的力必须通过直接的实际接触或机械接触才能产生。

Forces are transmitted into machine members through mating surfaces, e. g., from a gear to 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 forces at the boundaries or mating surfaces must be reasonable, and their intensities must be within the working limits of the materials composing the surfaces. For example, if the force operating on a journal bearing becomes too high, it will squeeze out the oil film and cause metal－to－metal contact, overheating, and rapid failure of the bearing. If the forces between gear teeth are too large, the oil film may be squeezed out from between them. This could result in flaking and spalling of the metal, noise, rough motion, and eventual failure. In the study of mechanics we are principally interested in determining the magnitude, direction, and location of the forces.

力通过接触表面传到机器中的各构件上。例如，从齿轮传到轴上或者从齿轮通过与其啮合的轮齿传到另一个齿轮，从V带传到带轮，或者从凸轮传到从动件。由于很多原因，人们必须知道这些力的大小。这些力在边界或在接触表面的分布必须合理，它们的大小必须在构成接触表面的材料的工作极限以内。例如，如果作用在一个滑动轴承上的力过大，它就会将油膜挤出，造成金属与金属的直接接触，产生过热和使轴承快速失效。如果齿轮相啮合的齿与齿之间的力过大，

就会将油膜从齿间挤压出来。这会造成金属表层的碎裂和剥落，使噪声增大，运动不精确，直至报废。在力学研究中，我们主要关心力的大小、方向和作用点。

Some of the terms used in mechanics are defined below.

在力学中要用到的一些术语定义如下：

Force：Our earliest ideas concerning forces arose because of our desire to push, lift, or pull various objects. So force is the action of one body on another. Our intuitive concept of force includes such ideas as place of application, direction, and magnitude, and these are called the characteristics of a force.

力关于力的最早概念是由于我们需要推、提或拉各种物体而产生的。因此，力是物体之间的相互作用。力的直观概念包括作用点、方向和大小，这些称为力的特性。

Mass is a measure of the quantity of matter that a body or an object contains. The mass of the body is not dependent on gravity and therefore is different from but proportional to its 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 amount of substance is called the mass of the rock.

质量是物体内所包含的物质的量。物体质量不受重力影响，因此与物体的重量不同，但是与其成正比。尽管一块月球岩石在月亮上和地球上的重量不同，但它的物质含量是不变的。这不变的物质含量就称为岩石的质量。

Inertia is the property of mass that causes it to resist any effort to change its motion.

惯性是质量所具有的抵抗任何外力改变其本身运动状态的性质。

Weight is the force with which a body is attracted to the earth or another celestial body, equal to the product of the object's mass and the acceleration of gravity.

重量是地球或其他天体对物体的作用力，它等于物体的质量与重力加速度的乘积。

Particle A particle is a body whose dimensions are so small that they may be neglected.

质点当一个物体的尺寸特别小，可以忽略不计时，该物体可以称为质点。

Rigid Body A rigid body does not change size and shape under the action of forces. Actually, All bodies are either elastic or plastic and will be 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. A rigid body is an idealization of a real body.

刚体刚体在受力后大小和形状都不会发生变化。实际上，所有的物体，不管是弹性体还是塑性体，在力的作用下都将发生变形。当物体的变形非常小时，为了简化计算，通常假设这个物体是刚体，也就是认为它没有发生变形。刚体是实际物体的理想化模型。

Deformable Body The rigid body assumption cannot 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 forces.

变形体在分析由于外力的作用所引起物体内部的应力和应变时，不能采用刚体假设。这时，我们认为物体能够变形。这样的分析通常称为弹性体分析，这时所用的假设为，在作用力的范围内，物体是弹性的。

Newton's Laws of Motion Newton's three laws are：

牛顿运动定律牛顿三定律为：

Law 1 If all the forces acting on a body are balanced, the body will either remain at rest or will continue to move in a straight line at a uniform velocity.

第一定律如果作用在一个物体上所有的力平衡，那么，这个物体将保持原来的静止或者匀速直线运动状态不变。

Law 2 If the forces acting on a body are not balanced, the body will experience an acceleration. The acceleration will be in the direction of the resultant force, and the magnitude of the acceleration will be proportional to the magnitude of resultant force and inversely proportional to the mass of body.

第二定律如果作用在一个物体上的那些力不平衡，那么，这个物体将产生加速度。加速度的方向与合力的方向相同；加速度的大小与合力的大小成正比，与物体的质量成反比。

Law 3 The forces of action and reaction between interacting bodies are equal in magnitude, opposite in direction, and have the same line of action.

第三定律相互作用的物体之间的作用力和反作用力大小相等，方向相反，作用在同一直线上。

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 mechanics are time, volume, density, speed, energy, and mass. Vector quantities, on the other hand, possess direction as well as magnitude. Examples of vectors are displacement, velocity, acceleration, force, moment, and momentum.

力学涉及两种类型的量：标量和矢量。标量是那些只有大小的量。在力学中标量的例子有时间、体积、密度、速率、能量和质量。另一方面，矢量既有大小又有方向。矢量的例子有位移、速度、加速度、力、力矩和动量。