物理学专业英语A1

1A1Motion,mass,andforcesNEWWORDSPage.6NEWWORDSPage.72A1Motion,mass,andforces•ScientistsmakemeasurementsusingSIunitssuchasthemetre,kilogram,second,andnewton.TheseandtheirabbreviationsarecoveredindetailinB1.However,youmayfinditeasiertoappreciatethelinksbetweendifferentunitsafteryouhavestudiedthewholeofsectionA.•Forsimplicity,unitswillbeexcludedfromsomestagesofthecalculationsinthisbook,asinthisexample:•totallength=2+3=5m•Strictlyspeaking,thisshouldbewritten:•totallength=2m+3m=5mUnitsofmeasurement3A1Motion,mass,andforces•Displacementisdistancemovedinaparticulardirection.TheSIunitofdisplacementisthemetre(m).•Quantities,suchasdisplacement,whichhavebothmagnitude(size)anddirection,arecalledvectors.••AB•Thearrowaboverepresentsthedisplacementofaparticlewhichmoves12mfromAtoB.However,withhorizontalorverticalmotion,itisoftenmoreconvenienttousea‘+’or‘-’toshowthevectordirection.Forexample:•Movementof12mtotheright:displacement=+12•Movementof12mtotheleft:displacement=-12Displacement12m4A1Motion,mass,andforces•Displacementisnotnecessarilythesameasdistancetravelled.Forexample,whentheballbelowhasreturnedtoitsstartingpoint,itsverticaldisplacementiszero.However,thedistancetravelledis10m.Displacement5mballthrownupfromhereballreturnstostartingpoint5A1Motion,mass,andforces•Averagespeediscalculatedlikethis:•distancetravelled•averagespeed=•timetaken•TheSIuintofspeedisthemetre/second,abbreviatedasms-1.Forexample,ifanobjecttravels12min2s,itsaveragespeedis6ms-1.•Averagevelocityiscalculatedlikethis:•displacement•averagespeed=•timetaken•TheSIunitofvelocityisalsothems-1.Butunlikespeed,velocityisavector.Speedandvelocity6A1Motion,mass,andforces•Thevelocityaboveisforaparticlemovingtotherightat6ms-1.However,aswithdisplacement,itisoftenmoreconvenienttousea‘+’or‘-’forthevectordirection.•Averagevelocityisnotnecessarilythesameasaveragespeed.Forexample,ifaballisthrownupwardsandtravelsatotaldistanceof10mbeforereturningtoitsstartingpoint2slater,itsaveragespeedis5ms-1.Butitsaveragevelocityiszero,becauseitsdisplacementiszero.Speedandvelocity6m7A1Motion,mass,andforces•Averageaccelerationiscalculatedlikethis:•changeinvelocity•averageacceleration=•timetaken•TheSIunitofaccelerationisthems-2(sometimeswrittenm/s2).Forexample,ifanobjectgain6ms-1ofvelocityin2s,itsaverageaccelerationis3ms-2.•Accelerationisavector.Theaccelerationvectoraboveisforaparticlewithanaccelerationof3ms-2totheright.However,aswithvelocity,itisoftenmoreconvenienttousea‘+’or‘-’forthevectordirection.Acceleration3ms-28A1Motion,mass,andforces•Ifvelocityincreasesby3ms-1eveysecond,theaccelerationis+3ms-2.Ifitdecreasesby3ms-1eveysecond,theaccelerationis-3ms-2.•Mathematically,anaccelerationof-3ms-2totherightisthesameasanaccelerationof+3ms-2totheleft.Acceleration9A1Motion,mass,andforces•Onthevelocity-timegraphabove,youcanworkouttheaccelerationovereachsectionbyfindingthegradientoftheline.Thegradientiscalculatedlikethis:•gainalongy-axis•gradient=•gainalongx-axisAcceleration66420timeinsvelocityinms-1acceleration=gradient=6/2=3ms-2acceleration=gradient=0/2=0ms-2acceleration=gradient=-6/2=-3ms-210•Forceisavector.TheSIisthenewton(N).•Iftwoormoreforcesactonsomething,theircombinedeffectiscalledtheresultantforce.Twosimpleexamplesareshownbelow.Intheright-handexample,theresultantforceiszerobecausetheforcesarebalanced.•Aresultantforceactingonamasscausesanacceleration.Theforce,mass,andaccelerationarelindedlikethis:•resultantforce=mass×accelerationF=ma•Forexample,a1Nresultantforcegivesa1kgmassanaccelerationof1ms-2.(Thenewtonisdefinedinthisway.)A1Motion,mass,andforcesForce11A1Motion,mass,andforces•Themoremasssomethinghas,themoreforceisneededtoproduceanygivenacceleration.•Whenbalancedforcesactonsomething,itsaccelerationiszero.Thismeansthatitiseitherstationaryormovingatasteadyvelocity(steadyspeedinastraightline).Force6N6N6N6NResultantforce=12NdownwardsResultantforce=012A1Motion,mass,andforces•OnEarth,everythingfeelsthedownwardforceofgravity.Thisgravitationalforceiscalledweight.Asforotherforces,itsSIunitisthenewton(N).•NeartheEarth’ssurface,thegravitationalforceoneachkgisabout10N:thegravitationalfieldstrengthis10Nkg-1.Thisisrepresentedbythesymbolg.Weightandgacceleration=weight/mass=10ms-2=gEarthmassweight1kg2kgm10N20Nmg13•Inthediagramabove,allthemassesarefallingfreely(gravityistheonlyforceacting).FromF=ma,itfollowsthatallthemasseshavethesamedownwardacceleration,g.Thisistheaccelerationoffreefall.•Youcanthinkofg:•Titherasagravitationalfieldstrengthof10Nkg-1•Orasanacceleratonoffreefallof10ms-2•Inmoreaccuratecalculation,thevalueofgisnormallybakentobe9.81,ratherthan10.A1Motion,mass,andforcesForce14•Theturningeffectofforceiscalledamoment:•momentofforce=force×perpendiculardistance*•aboutapointfrompoint•*measuredfromthelineofactionoftheforce.•Thedumb-bellbelowbalancesatpointObecausethetwomomentsaboutOareequalbutopposite.A1Motion,mass,andforcesMomentsandbalance6N2m4m3NanticlockwisemomentaboutO=6×2=12NmclockwisemomentaboutO=3×4=12Nm15•Thedumb-bellismadeupofsmallerparts,eachwithitsownweight.Together,theseareequivalenttoasinglef