机械工程导论（双语）PPT课件.pptx

LOGOmechanical engineering An introduction to1Lecturer: Liu Ju-rongCHAPTER2 Problem-Solving SkillsOVERVIEW1UNIT SYSTEMS AND CONVERSIONS23DIMENSIONAL CONSISTENCY 2SIGNIFICANT DIGITS4AN ERROR OF UNITS ON THE WAY TO MAR5APPROXIMATION IN ENGINEERING6PROBLEM-SOLVING METHODOLOGY7Vocabulary3Torque Torque 扭矩扭矩thermal conductivity thermal conductivity 热导率热导率shear stress shear stress 剪应力剪应力fluid viscosity fluid viscosity 流体的粘度流体的粘度elastic modulus elastic modulus 弹性模量弹性模量kinetic energy kinetic energy 动能动能Reynolds number Reynolds number 雷诺数雷诺数1 OVERVIEW-the important of unit4How important a unit is?Each quantity in mechanical 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 chapter5After study, what should you be able to have?Report both 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 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 order-of-magnitude approximations. 2 UNIT SYSTEMS AND CONVERSIONS- Unit System what is USCS and SI?6Engineers 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 conversant with both unit systems. 2 UNIT SYSTEMS AND CONVERSIONS- an example Keeping Track of Units on Fright 1437In 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), mass (kg)Total fuel22,300 kilograms (kg) How much fuel should be added?Fuel in tanks7682 liters (L)Boeing 767 pounds (lb); kilograms1.77 lb/L1.77 kg/L9000 liters16,000 liters 2 UNIT SYSTEMS AND CONVERSIONS- Base and Derived Units what is base units? What is derived units?8A base unit is a fundamental quantity that cannot be further 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.92 WHAT IS ENGINEERING?-United States Customary System The United States Customary System of units is a historical and traditional system, and its origin traces back to the ancient Roman Empire.Why does the United States stand out in retaining the USCS? 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 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. 102 WHAT IS ENGINEERING?-United States Customary System QuantityUSCS Base UnitAbbreviationLengthfootftForcepoundlbTimesecondsElectric currentampereAThermodynamic temperatureRankineRAmount of substancemolemolLuminous intensitycandelacdTABLE 2.1 Base Units in the USCS112 WHAT IS ENGINEERING?-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.QuantityUSCS Derived UnitAbbreviationDefinitionLengthmilmil1 mil = 0.001 in.Lengthinchin.1 in. = 0.0833ftLengthyardyd1 yd = 3 ftLengthmilemi1 mi = 5280 ftVolumeU.S. gallongal1 gal = 0.1337 ft3Massslugslug1 slug = 1 lbs2/ftForceounceoz1 oz = 0.0625 lbForcetonton1 ton = 2000lbMoment of a forceftlbftlb-Pressure or stresspsipsi1 psi = 1 lb/in2Energy, work, or heatftlbftlb-Energy, work, or heatBtuBtu1 Btu = 778.2 ftlbPowerhorsepowerhp1 hp = 550 ftlb/sTemperaturedegree FahrenheitFF = R - 460 122 WHAT IS ENGINEERING?-International System of Units The International System of units (or SI) is the measurement standard based in part on the quantities of meters, kilograms, and seconds. QuantitySI Base UnitAbbreviationLengthmetermForcekilogramkgTimesecondsElectric currentampereAThermodynamic temperatureKelvinKAmount of substancemolemolLuminous intensitycandelacdTABLE 2.3 Base Units in the SI132 WHAT IS ENGINEERING?-International System of Units QuantitySI Derived UnitAbbreviationdefinitionLengthmicronm1 m = 10-6 mVolumeliterL1 L =0.001 m3ForcenewtonN1 N = 1kgm/s2Moment of a forceNmNm-Pressure or stresspascalPa1 Pa = 1 N/m2 Energy, work, or heatjouleJ1 J = 1 NmPowerwattW1 W = 1 J temperaturedegree Celsius = K - 273 TABLE 2.4 Certain Derived in the SI. Although a change in temperature of 1 Kelvin also equals a change of 1 degree Celsius, the absolute values are converted using the formula.142 WHAT IS ENGINEERING?-International System of Units TABLE 2.5 onversion Factor Between Certain Quantities in the USCS and SIQuantityTo Convert from To Multipy by Lengthfoot (ft)meter (m)0.3048Lengthinch (in.)meter (m)0.0254Lengthmile (mi)kilometer (km)1.609Volumegallon (gal)meter3 (m3)3.78510-3Volumegallon (gal)liter (L)3.875Massslugkilogram (kg)14.59Forcepound (lb)newton (N)4.448Pressure or stresspound/inch2 (psi)pascal (Pa)6895Energy, work, or heatfoot-pound (ftlb)joule (J)1.356Powerfoot-pound/second (ftlb/s)watt (W)1.356Powerhorsepower (hp)kilowatt (kW)0.7456152 WHAT IS ENGINEERING?-International System of Units TABLE 2.6 onversion Factor Between Certain Quantities in the SI and USCSQuantityTo Convert from To Multipy by Lengthmeter (m)foot (ft)0.3048Lengthmeter (m)inch (in.)0.0254Lengthkilometer (km)mile (mi)1.609Volumemeter3 (m3)gallon (gal)3.78510-3Volumeliter (L)gallon (gal)3.875Masskilogram (kg)slug14.59Forcenewton (N)pound (lb)4.448Pressure or stresspascal (Pa)pound/inch2 (psi)6895Energy, work, or heatjoule (J)foot-pound (ftlb)1.356Powerwatt (W)foot-pound/second (ftlb/s)1.356Powerkilowatt (kW)horsepower (hp)0.7456162 WHAT IS ENGINEERING?-International System of Units TABLE 2.6 onversion Factor Between Certain Quantities in the SI and USCSNamesymbolMultiplicative FactorteraT1,000,000,000,000 = 1012gigaG1,000,000,000 = 109megaM1,000,000 = 106kilok1000 = 103hectoh100 = 102decada10 = 101decid0.1 = 10-1centic0.01 = 10-2millim0.001 = 10-3micro0.000,001 = 10-6nanon0.000,000,001 = 10-9picop0.000,000,000,001 = 10-123 DIMENSIONAL DIMENSIONAL CONSISTENCYCONSISTENCY17Dimensional consistency means that the units associated with the numerical values on each side of the equality 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 SIGNIFICANT DIGITSDIGITS18 A significant digit is a numerical value that is known to be correct 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 factor 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 mN43.01 mN 43.02 mN5 AN ERROR OF UNITS ON THE WAY TO MARS19The importance of keeping track of units in engineering calculations was highlighted by 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 identified eight factors that contributed to the spacecrafts loss. units of (force) (time)newton-secondspound-seconds 6 APPROXIMATION IN ENGINEERING20Engineers are comfortable making reasonable approximations so that the systems they analyze are as simple as possible and yet will yield a result 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 IN ENGINEERING21Given 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 METHODOLOGY22 1.Make a clean start 2. Draw 3. Givens and unknowns. 4. Think first, then write. 5. Be coordinated238. Significant figures 9. Box it 10. Interpret your answer 6. Neatness counts. 7. Units. 7 PROBLEM-SOLVING METHODOLOGYSUMMARY24 Engineers are often described as being can-do people with excellent problem- solving skills. In this chapter we have discussed some of the fundamental tools and skills that mechanical engineers use when they answer technical questions. Numerical values, the USCS and SI systems, unit conversions, dimensional consistency, significant digits, and order-of-magnitude approximations are everyday issues for engineers. Each quantity in mechanical engineering has two components- a numerical value and a unit-and it is meaningless to report one without the other. Mechanical engineers need to be clear about those numerical values and units when they perform calculations, document results, and communicate their findings to others through written reports and oral presentations. By following the consistent problem-solving guidelines developed in this chapter, you will be able to approach engineering problems in a systematic manner and be confident of the accuracy of your solutions.SELF-STUDY AND REVIEW25What should you review?What are the base units 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.LOGO26