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 mot a factor, and dynamics deals with systems which change with time.
  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.
  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.
  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.
reaction in the shaft  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 f the couple is the plane containing the two lines of action.
  Some of the terms used in mechanics are defined below.
  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.
Matter is any material or substance; if it is completely enclosed, it is called a body.
  Inertia is the property of mass that causes it to resist any effort to change its motion.
  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.
《机械工程专业英语教程》 电子工业出版社  施平
力学基本概念
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，动力学对承受时间而变化的系统进行分析。
当一些物体联接在一起形成一个组合体或者系统时，任何两个相联接的物体之间的作用和反作用力被称为约束力。这些力约束着各个物体，使其牌特定的状态。从外部施加到这个物体系统的力被称为外力。
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则必须通过直接的实际接触或机械接触才能产生。
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轮齿传到另一个齿轮，从三角皮带传到皮带轮，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的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，造成金属与金属的直接接触，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的力过大，就会将油膜从齿间挤压出来。这会造成金属的碎裂和剥落，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。
作用在同一个刚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并成一个合力。作用在一个刚体上的两个这样的力构成一个力偶。力偶臂是这两条作用线之间的垂直距离，力偶面是包含这两条作用线的平面。
在力学中要用到的一些术语定义如下。
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，力是物体之间相互作用。力的直观概念包括作用点、方向和大小，这些被穆尔为力的要素。
物质  物质是任何霉烂或实体，如果它是全封闭的，内里称为物体。
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。
重量  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。

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