Basic Concepts in Mechanics力学基本概念 机械工程专业英语第一课.doc

Basic Concepts in Mechanics The branch of scientific analysis which deals with motions, time, and force 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. Forces are transmitted into machine members through mating surfaces, e.g., 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 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. 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 in 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. 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 subst