1、mechanical engineering,An introduction to,1,Lecturer: Liu Ju-rong,CHAPTER2 Problem-Solving Skills,2,Vocabulary,3,Torque 扭矩 thermal conductivity 热导率 shear stress 剪应力 fluid viscosity 流体的粘度 elastic modulus 弹性模量 kinetic energy 动能 Reynolds number 雷诺数,1 OVERVIEW-the important of unit,4,Each quantity in me
2、chanical engineering has two components: a numerical value and a unit.,One is simply meaningless without the other, and practicing engineers pay as close and careful attention to the units in a calculation as they do to the numbers.,1 OVERVIEW- The ability to be available in this chapter,5,Report bo
3、th a numerical value and its unit in each calculation that you perform.,List the base units in the United States Customary System and the System International d Units, and state some of the derived units used in mechanical engineering.,Understand the need for proper bookkeeping of units when making
4、engineering calculations and the implications of not doing so.,Convert numerical quantities from the United States Customary System to the System International d Units, and vice versa.,Check your equations and calculations to verify that they are dimensionally consistent.,Understand how to perform o
5、rder-of-magnitude approximations.,2 UNIT SYSTEMS AND CONVERSIONS- Unit System,6,Engineers specify physical quantities in two different, but conventional, systems of units: the United States Customary System (USCS) and the International System of Units. Practicing mechanical engineers must be convers
6、ant with both unit systems.,2 UNIT SYSTEMS AND CONVERSIONS- an example,7,In July of 1983, Air Canada Flight 143 was en route from Montreal to Edmonton. The Boeing 767 had three fuel tanks, one in each wing and one in the fuselage, which supplied the planes two jet engines.,volume (L), weight (lb), m
7、ass (kg),Total fuel22,300 kilograms (kg),How much fuel should be added ?,Fuel in tanks 7682 liters (L),Boeing 767,pounds (lb);,kilograms,1.77 lb/L,1.77 kg/L,9000 liters,16,000 liters,2 UNIT SYSTEMS AND CONVERSIONS- Base and Derived Units,8,A base unit is a fundamental quantity that cannot be further
8、 broken down or expressed in terms of any simpler elements. Base units are the core building blocks of any unit system.,Derived units, as their name implies, are constructed as combinations of base units.,9,2 WHAT IS ENGINEERING?-United States Customary System,The United States Customary System of u
9、nits is a historical and traditional system, and its origin traces back to the ancient Roman Empire.,The reasons are both logistical and cultural: There is already a vast continent-sized infrastructure within the United States that is based on the USCS.,Conversion away from the existing system would
10、 be a significant and expensive burden.,The dimensions of countless existing structures, factories, machines, and spare parts have already been specified and built in terms of the USCS.,10,2 WHAT IS ENGINEERING?-United States Customary System,TABLE 2.1 Base Units in the USCS,11,2 WHAT IS ENGINEERING
11、?-United States Customary System,TABLE 2.2 Certain Derived in the USCS. Although a change in temperature of 1 Rankine also equals a change of 1 degree Fahrenheit, the absolute values are converted using the formula.,12,2 WHAT IS ENGINEERING?-International System of Units,The International System of
12、units (or SI) is the measurement standard based in part on the quantities of meters, kilograms, and seconds.,TABLE 2.3 Base Units in the SI,13,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.4 Certain Derived in the SI. Although a change in temperature of 1 Kelvin also equals a change o
13、f 1 degree Celsius, the absolute values are converted using the formula.,14,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.5 onversion Factor Between Certain Quantities in the USCS and SI,15,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.6 onversion Factor Between Certain
14、 Quantities in the SI and USCS,16,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.6 onversion Factor Between Certain Quantities in the SI and USCS,3 DIMENSIONAL CONSISTENCY,17,Dimensional consistency means that the units associated with the numerical values on each side of the equality
15、sign match.,In paper-and-pencil calculations, it is good practice to keep the units adjacent to each numerical quantity in an equation.,performing a double-check on the units in any equation is always a good idea.,4 SIGNIFICANT DIGITS,18,A significant digit is a numerical value that is known to be c
16、orrect and reliable in the light of inaccuracy that is present in the supplied information, any approximations that have been made along the way, and the mechanics of the calculation itself.,The precision of a number is half as large as the last significant digit used in expressing the number.,The f
17、actor of one-half( arises because me last digit of a number represents the rounding-off process either higher or lower, of the trailing digits.,43.01 mN,43.01 mN,43.02 mN,5 AN ERROR OF UNITS ON THE WAY TO MARS,19,The importance of keeping track of units in engineering calculations was highlighted by
18、 the failure of the Mars Climate Orbiter spacecraft in 1999.,The spacecraft was to arrive at Mars on September 23, 1999, and it was scheduled to complete its primary science mission on December 31, 2004.,NASA conducted a thorough investigation, and the Mars Climate Orbiter Mishap Investigation Board
19、 identified eight factors that contributed to the spacecrafts loss.,units of (force) (time),newton-seconds,pound-seconds,6 APPROXIMATION IN ENGINEERING,20,Engineers are comfortable making reasonable approximations so that the systems they analyze are as simple as possible and yet will yield a result
20、 that is accurate enough for the task at hand.,Engineers nearly always make approximations when they design and solve technical problems.,Approximations are also useful to remove extraneous features that complicate the problem but otherwise have little influence on the final answer.,6 APPROXIMATION
21、IN ENGINEERING,21,Given the uncertainty present in real systems, it is often necessary for engineers to make “order-of-magnitude“ approximations.,These are sometimes called “back-of-the-envelope“ calculations because they can be performed quickly and informally.,7 PROBLEM-SOLVING METHODOLOGY,22,1.Ma
22、ke a clean start,2. Draw,3. Givens and unknowns.,4. Think first, then write,5. Be coordinated,23,8. Significant figures,9. Box it,10. Interpret your answer,6. Neatness counts.,7. Units.,7 PROBLEM-SOLVING METHODOLOGY,SUMMARY,24,SELF-STUDY AND REVIEW,25,What should you review?,What are the base units
23、in the USCS and SI?,What are the examples of derived units in the USCS and SI?,How are mass and force treated in the USCS and SI?,How should you decide the number of significant digits to retain in a calculation an report in your final answer?,Summarize the major steps that should be followed when solving problems to clearly document your work and to catch otherwise avoidable mistakes.,26,Thank You !,
