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1、PhysicsProfessionalEnglish物理学专业英语PhysicsProfessionalEnglishA1Motion,mass,andforcesNEWWORDSPage.6NEWWORDSPage.71A1Motion,mass,andforcesScientistsmakemeasurementsusingSIunitssuchasthemetre,kilogram,second,andnewton.TheseandtheirabbreviationsarecoveredindetailinB1.However,youmayfinditeas
2、iertoappreciatethelinksbetweendifferentunitsafteryouhavestudiedthewholeofsectionA.Forsimplicity,unitswillbeexcludedfromsomestagesofthecalculationsinthisbook,asinthisexample:totallength=2+3=5mStrictlyspeaking,thisshouldbewritten:totallength=2m+3m=5mUnitsofmeasurement2A1Motion,mass,andf
3、orcesDisplacementisdistancemovedinaparticulardirection.TheSIunitofdisplacementisthemetre(m).Quantities,suchasdisplacement,whichhavebothmagnitude(size)anddirection,arecalledvectors.ABThearrowaboverepresentsthedisplacementofaparticlewhichmoves12mfromAtoB.However,withhorizontalorvertical
4、motion,itisoftenmoreconvenienttousea‘+’or‘-’toshowthevectordirection.Forexample:Movementof12mtotheright:displacement=+12Movementof12mtotheleft:displacement=-12Displacement12m3A1Motion,mass,andforcesDisplacementisnotnecessarilythesameasdistancetravelled.Forexample,whentheballbelowhasre
5、turnedtoitsstartingpoint,itsverticaldisplacementiszero.However,thedistancetravelledis10m.Displacement5mballthrownupfromhereballreturnstostartingpoint4A1Motion,mass,andforcesAveragespeediscalculatedlikethis:distancetravelledaveragespeed=timetakenTheSIuintofspeedisthemetre/second,abbrev
6、iatedasms-1.Forexample,ifanobjecttravels12min2s,itsaveragespeedis6ms-1.Averagevelocityiscalculatedlikethis:displacementaveragespeed=timetakenTheSIunitofvelocityisalsothems-1.Butunlikespeed,velocityisavector.Speedandvelocity5A1Motion,mass,andforcesThevelocityaboveisforaparticlemovingto
7、therightat6ms-1.However,aswithdisplacement,itisoftenmoreconvenienttousea‘+’or‘-’forthevectordirection.Averagevelocityisnotnecessarilythesameasaveragespeed.Forexample,ifaballisthrownupwardsandtravelsatotaldistanceof10mbeforereturningtoitsstartingpoint2slater,itsaveragespeedis5ms-1.Buti
8、tsave
