Basic Concepts in Mechanics[mi’kæniks] 第一课力学基本概念The branch of scientific analysis [ə’næləsis] 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、时间和作用力作出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的系统进行分析。
[扩展1]：静力学是力学的一个分支，它主要研究物体在力的作用下处于平衡的规律，以及如何建立各种力系的平衡条件。
平衡是物体机械运动的特殊形式，严格地说，物体相对于惯性参照系处于静止或作匀速直线运动的状态，即加速度为零的状态都称为平衡。
静力学在工程技术中有着广泛的应用。
例如对房屋、桥梁的受力分析，有效载荷的分析计算等。
[扩展2]：动力学是理论力学的一个分支学科，它主要研究作用于物体的力与物体运动的关系。
动力学的研究对象是运动速度远小于光速的宏观物体。
动力学是物理学和天文学的基础，也是许多工程学科的基础。
对动力学的研究使人们掌握了物体的运动规律，并能够为人类进行更好的服务。
例如，牛顿发现了万有引力定律，解释了开普勒定律，为近代星际航行，发射飞行器考察月球、火星、金星等等开辟了道路。
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 anther gear, from a connecting rod 连杆through a bearing to a lever, from a V belt to a pulley[‘puli]滑轮、皮带轮，or from a cam 凸轮[kæm] to a follower从动件.力通过配合表面(啮合面)传到机器中的各构件上。
例如，从齿轮传到轴或者从齿轮通过啮合的轮齿传到另一齿轮，从连杆通过轴承传到另一杆件，从三角皮带传到皮带轮，或者从凸轮传到从动件。
[扩展3]：mate 和mesh。
mate [❍♏♓♦] n.配偶, 对手, 助手；vt.使配对, 使一致, 结伴；vi.成配偶, 紧密配合，使啮合。
mesh[❍♏☞] n.网孔, 网丝, 网眼, 圈套, 陷阱, [机]啮合vt.以网捕捉, 啮合, 编织vi.落网, 相啮合。
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 sleeve bearing becomes too high, it will squeeze out the oil film薄膜and cause metal－to－metal contact, overheating，and rapid failure of the bearing轴承．If the forces between gear teeth are too large, the oil film may be squeezed out from between them．This could result in flaking剥落and spalling碎裂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．由于很多原因,人们必须知道这些力的大小。
这些力在边界或在配合表面(啮合面)的分布必须合理，它们的太小必须在构成配合表面(啮合面)的材料的工作极限以内。
例如，如果作用在一个套筒轴承上的力太大，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之间的力过大，就会将油膜从齿间挤压出来。
这会造成金属的剥落和碎裂，噪音增大，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 this phase [feiz]of our studies are defined below．我们将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 acting 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、举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。
这些称为力的要素。
Matter Matter is any material or substance；if it is completely enclosed，it is called a body．物质物质是任何材料或实体，如果它是全封闭的，则称为物体。
Mass Newton[‘nju:tn]defined mass as the quantity of matter of a body as measured by its volume and density．This is not a very satisfactory definition because density is the mass of a unit volume．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 amount of substance, or quantity of matter，is called the mass of the rock.质量牛顿将质量定义为用物体的体积和密度来量度的、物体所含物质的量。
因为密度是单位体积的质量，所以这不是一个很令人满意的定义。
我们估计当时牛顿大概没有意识到他在下定义，因而我们可以原谅他。
不过，他认识到这样一个事实，一切物体都具有某种与重量不同的内在性质。
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[ ] Inertial is the property of mass that causes it to resist any effort to change its motion．惯性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 Weight is the force of gravity acting upon a mass. The following quotation is pertinent:重量重量是作用在质量上的重力。
下列引文确切地论述了质量和重量的关系：The great advantage of SI (International System of Units) units is that there is one，and only one unit for each physical quotation—the meter for length, the kilogram for mass，the Newton for force，the second for time，etc．To be 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”has 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.国际标准单位制的最大优越性是，对每一个物理量有一个，而且只有一个单位——如长度单位为米，质量单位为千克，力的单位为牛顿，时间单位为秒等。