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1. The fundamental principle involved is the use of compressed air acting through a piston in a cylinder to set block brakes on the wheels. The action is simultaneous on the wheels of all the cars in the train. The compressed air is carried through a strong hose from car to car with couplings between cars; its release to all the separate block brake units, at the same time, is controlled by the engineer. (Braking Systems)相关的基本原理是使用压缩气体，通过气缸内的活塞将闸块压在车轮起作用。
列车的所有车厢上的车轮同时动作。
压缩气体通过一个坚固的管道在由联轴器连接的车厢之间传输；工程师控制所有独立闸块单元的同时释放。
2. When the brake pedal of an automobile is depressed, a force is applied to a piston in a master cylinder. The piston forces hydraulic fluid through metal tubing into a cylinder in each wheel where the fluid’s pressure moves two pisto ns that press the brake shoes against the drum. (Braking Systems)当踩下汽车刹车的踏板，施加一个力在主汽缸中的活塞上。
2 problem-solving skills2.1 OVERVIEWIn this chapter we begin discussing some of the steps that engineers follow when they solve problems and perform calculations in their daily work .Mechanical engineers are fluent with numbers,and they are organized when obtaining numerical answers to questions that involve a remarkable breadth of variables and physical properties.Some of the questions that you will encounter in your study of mechanical engineering are force,torque,thermal conductivity,shear stress,fluid viscosity[vi'skɔsəti] n. [物] 粘性，[物] 粘度,elastic modulus ,kinetic energy,Reynolds['renəldz] n. 雷诺兹（姓氏）number,specific heat,and so on.The list is very long indeed.The only way that you will make sense of so many quantities is to be very clear about them in calculations and when explaining results to others.Each quantity in mechanical engineering has two components :a numerical value and a unit.One is simply meaningless without others,and practicing engineers pay as close and careful attention to the units in a calculation as they do to the numbers.In the first portion of this chapter,we will discuss fundamental concepts for systems of units and conversions between them and a procedure for checking dimensional consistency that will serve you well in engineering calculations.Aside from the question of units,engineers must obtain numerical answers to questions- how strong ？how heavy？how much power?what temperature?—often in the face of uncertainty and incomplete information.At the start of a new design,for instance,the shape and dimensions of the final product are not known ;if they were ,then it wouldn,t be necessary to design in the first place .Only rough estimates of the force applied to a structure might be available.Likewise,exact values for material properties are rarely known,and there will always be some variation between samples of materials.Nevertheless,an engineer still has a job to do,and the design process must start somewhere.For that reason,engineer make approximations in order to assign numerical values to quantities that are otherwise unknown. Those approximations are understood to be imperfect,but they are better than random guesses and certainly better than nothing.Mechanical engineer use their common sense,experience,intuition,judgment,and physical laws to find answers through a process called order-of-magnitude approximation.These estimates are a first step toward reducing a concrete physical situation to its most essential and relevant pieces.Mechanical engineers are comfortable making reasoned approximations.In the following sections we begin a discussion of numerical values,unit systems,significant digits,and approximation.These principles and techniques will be applied further as we explore the"element"of mechanical engineering outlined in later chapter. After completing this chapter,you should be able to:● Report both a numerical value and its units in each calculationthat you perform.● List the base units in the United States Customary System andthe Systeme International d'unites,and state some of the derived units used in mechanical engineering.● Understand the need for proper bookkeeping of units whenmaking engineering calculations and the implications of not doing so.●Convert numerical quantities from the United States CustomarySystem to the Systeme International d'Unites,and vice versa.● Check your equations and calculations to verify that they aredimensionally consistent.● Understand how to perform order-of-magnitude approximations. 第二章解决问题的技能2.1概述这一章，我们开始探讨机械工程师在日常工作中，解决问题、进行计算时所采用的步骤。
