机械工程专业英语教程(第2版)[施平主编][翻译]_lesson1

Basic Concept in Mechanics

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、时间和作用力做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，任何两个相连接的物体之间的作用力和反作用力被称为约束力。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的系统的力被称为外力。

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的力的例子。与我们有关的许多力都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 .

力通过配合表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齿轮传到另一个齿轮；从V带传到皮带轮；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。这些力在边界或在配合表面的分布必须合理，它们的大小必须在构成配合表面材料的工作极限以内。例如，作用在一个滑动轴承上的力太大，它就会将油膜挤出，造成金属与金属直接接触，产生过热和使轴承快速失效。如果齿轮间相啮合的齿与齿之间力过大，就会将油膜从齿间挤压出来。这会造成表层金属碎裂和剥落，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。

Two equal and opposite forces acting along two non-coincident parallel straight lines in a body cannot be combined to obtain a single resultant force . Any two such forces acting on a body constitute a couple . The arm 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 .

作用在同一个物体上的两条不重合的平行线上的两个大小相等、方向相反的力不能被合