机械工程导论(双语)课件2

LOGOmechanicalengineeringAnintroductionto1Lecturer:LiuJu-rongCHAPTER2Problem-SolvingSkillsOVERVIEW1UNITSYSTEMSANDCONVERSIONS23DIMENSIONALCONSISTENCY2SIGNIFICANTDIGITS4ANERROROFUNITSONTHEWAYTOMAR5APPROXIMATIONINENGINEERING6PROBLEM-SOLVINGMETHODOLOGY7Vocabulary3Torque扭矩thermalconductivity热导率shearstress剪应力fluidviscosity流体的粘度elasticmodulus弹性模量kineticenergy动能Reynoldsnumber雷诺数1OVERVIEW----theimportantofunit4Howimportantaunitis?Eachquantityinmechanicalengineeringhastwocomponents:anumericalvalueandaunit.Oneissimplymeaninglesswithouttheother,andpracticingengineerspayascloseandcarefulattentiontotheunitsinacalculationastheydotothenumbers.1OVERVIEW---Theabilitytobeavailableinthischapter5Afterstudy,whatshouldyoubeabletohave?Reportbothanumericalvalueanditsunitineachcalculationthatyouperform.ListthebaseunitsintheUnitedStatesCustomarySystemandtheSystemInternationald‘Units,andstatesomeofthederivedunitsusedinmechanicalengineering.Understandtheneedforproperbookkeepingofunitswhenmakingengineeringcalculationsandtheimplicationsofnotdoingso.ConvertnumericalquantitiesfromtheUnitedStatesCustomarySystemtotheSystemInternationald‘Units,andviceversa.Checkyourequationsandcalculationstoverifythattheyaredimensionallyconsistent.Understandhowtoperformorder-of-magnitudeapproximations.2UNITSYSTEMSANDCONVERSIONS-UnitSystemwhatisUSCSandSI?6Engineersspecifyphysicalquantitiesintwodifferent,butconventional,systemsofunits:theUnitedStatesCustomarySystem(USCS)andtheInternationalSystemofUnits.Practicingmechanicalengineersmustbeconversantwithbothunitsystems.2UNITSYSTEMSANDCONVERSIONS-anexampleKeepingTrackofUnitsonFright1437InJulyof1983,AirCanadaFlight143wasenroutefromMontrealtoEdmonton.TheBoeing767hadthreefueltanks,oneineachwingandoneinthefuselage,whichsuppliedtheplane'stwojetengines.volume(L),weight(lb),mass(kg)Totalfuel22,300kilograms(kg)Howmuchfuelshouldbeadded?Fuelintanks7682liters(L)Boeing767pounds(lb);kilograms1.77lb/L1.77kg/L9000liters16,000liters2UNITSYSTEMSANDCONVERSIONS-BaseandDerivedUnitswhatisbaseunits?Whatisderivedunits?8Abaseunitisafundamentalquantitythatcannotbefurtherbrokendownorexpressedintermsofanysimplerelements.Baseunitsarethecorebuildingblocksofanyunitsystem.Derivedunits,astheirnameimplies,areconstructedascombinationsofbaseunits.92WHATISENGINEERING?---UnitedStatesCustomarySystemTheUnitedStatesCustomarySystemofunitsisahistoricalandtraditionalsystem,anditsorigintracesbacktotheancientRomanEmpire.WhydoestheUnitedStatesstandoutinretainingtheUSCS?Thereasonsarebothlogisticalandcultural:Thereisalreadyavastcontinent-sizedinfrastructurewithintheUnitedStatesthatisbasedontheUSCS.Conversionawayfromtheexistingsystemwouldbeasignificantandexpensiveburden.Thedimensionsofcountlessexistingstructures,factories,machines,andsparepartshavealreadybeenspecifiedandbuiltintermsoftheUSCS.102WHATISENGINEERING?---UnitedStatesCustomarySystemQuantityUSCSBaseUnitAbbreviationLengthfootftForcepoundlbTimesecondsElectriccurrentampereAThermodynamictemperatureRankineRAmountofsubstancemolemolLuminousintensitycandelacdTABLE2.1BaseUnitsintheUSCS112WHATISENGINEERING?---UnitedStatesCustomarySystemTABLE2.2CertainDerivedintheUSCS.Althoughachangeintemperatureof1Rankinealsoequalsachangeof1degreeFahrenheit,theabsolutevaluesareconvertedusingtheformula.QuantityUSCSDerivedUnitAbbreviationDefinitionLengthmilmil1mil=0.001in.Lengthinchin.1in.=0.0833ftLengthyardyd1yd=3ftLengthmilemi1mi=5280ftVolumeU.S.gallongal1gal=0.1337ft3Massslugslug1slug=1lb·s2/ftForceounceoz1oz=0.0625lbForcetonton1ton=2000lbMomentofaforceft·lbft·lb-------------Pressureorstresspsipsi1psi=1lb/in2Energy,work,orheatft·lbft·lb-------------Energy,work,orheatBtuBtu1Btu=778.2ft·lbPowerhorsepowerhp1hp=550ft·lb/sTemperaturedegreeFahrenheit˚F˚F=˚R-460122WHATISENGINEERING?---InternationalSystemofUnitsTheInternationalSystemofunits(orSI)isthemeasurementstandardbasedinpartonthequantitiesofmeters,kilograms,andseconds.QuantitySIBaseUnitAbbreviationLengthmetermForcekilogramkgTimesecondsElectriccurrentampereAThermodynamictemperatureKelvinKAmountofsubstancemolemolLuminousintensitycandelacdTABLE2.3BaseUnitsintheSI132WHATISENGINEERING?---InternationalSystemofUnitsQuantitySIDerivedUnitAbbreviationdefinitionLengthmicronμm1μm=10-6mVolumeliterL1L=0.001m3ForcenewtonN1N=1kg·m/s2MomentofaforceN·mN·m-----------PressureorstresspascalPa1Pa=1N/m2Energy,work,orheatjouleJ1J=1N·mPowerwattW1W=1JtemperaturedegreeCelsius℃℃=˚K-273TABLE2.4CertainDerivedintheSI.Althoughachangeintemperatureof1Kelvinalsoequalsachangeof1degreeCelsius,theabsolutevaluesareconvertedusingtheformula.142WHATISENGINEERING?---InternationalSystemofUnitsTABLE2.5onversionFactorBetweenCertainQuantitiesintheUSCSandSIQuantityToConvertfrom···To···Multipyby···Lengthfoot(ft)meter(m)0.3048Lengthinch(in.)meter(m)0.0254Lengthmile(mi)kilometer(km)1.609Volumegallon(gal)meter3(m3)3.785×10-3Volumegallon(gal)liter(L)3.875Massslugkilogram(kg)14.59Forcepound(lb)newton(N)4.448Pressureorstresspound/inch2(psi)pascal(Pa)6895Energy,work,orheatfoot-pound(ft·lb)joule(J)1.356Powerfoot-pound/second(ft·l