物理学专业英语A1ppt课件

1,PhysicsProfessionalEnglish,物理学专业英语,PhysicsProfessionalEnglish,A1Motion,mass,andforces,NEWWORDSPage.6,NEWWORDSPage.7,2,A1Motion,mass,andforces,ScientistsmakemeasurementsusingSIunitssuchasthemetre,kilogram,second,andnewton.TheseandtheirabbreviationsarecoveredindetailinB1.However,youmayfinditeasiertoappreciatethelinksbetweendifferentunitsafteryouhavestudiedthewholeofsectionA.Forsimplicity,unitswillbeexcludedfromsomestagesofthecalculationsinthisbook,asinthisexample:totallength=2+3=5mStrictlyspeaking,thisshouldbewritten:totallength=2m+3m=5m,Unitsofmeasurement,3,A1Motion,mass,andforces,Displacementisdistancemovedinaparticulardirection.TheSIunitofdisplacementisthemetre(m).Quantities,suchasdisplacement,whichhavebothmagnitude(size)anddirection,arecalledvectors.ABThearrowaboverepresentsthedisplacementofaparticlewhichmoves12mfromAtoB.However,withhorizontalorverticalmotion,itisoftenmoreconvenienttousea+or-toshowthevectordirection.Forexample:Movementof12mtotheright:displacement=+12Movementof12mtotheleft:displacement=-12,Displacement,12m,4,A1Motion,mass,andforces,Displacementisnotnecessarilythesameasdistancetravelled.Forexample,whentheballbelowhasreturnedtoitsstartingpoint,itsverticaldisplacementiszero.However,thedistancetravelledis10m.,Displacement,5m,ballthrownupfromhere,ballreturnstostartingpoint,5,A1Motion,mass,andforces,Averagespeediscalculatedlikethis:distancetravelledaveragespeed=timetakenTheSIuintofspeedisthemetre/second,abbreviatedasms-1.Forexample,ifanobjecttravels12min2s,itsaveragespeedis6ms-1.Averagevelocityiscalculatedlikethis:displacementaveragespeed=timetakenTheSIunitofvelocityisalsothems-1.Butunlikespeed,velocityisavector.,Speedandvelocity,6,A1Motion,mass,andforces,Thevelocityaboveisforaparticlemovingtotherightat6ms-1.However,aswithdisplacement,itisoftenmoreconvenienttousea+or-forthevectordirection.Averagevelocityisnotnecessarilythesameasaveragespeed.Forexample,ifaballisthrownupwardsandtravelsatotaldistanceof10mbeforereturningtoitsstartingpoint2slater,itsaveragespeedis5ms-1.Butitsaveragevelocityiszero,becauseitsdisplacementiszero.,Speedandvelocity,6m,7,A1Motion,mass,andforces,Averageaccelerationiscalculatedlikethis:changeinvelocityaverageacceleration=timetakenTheSIunitofaccelerationisthems-2(sometimeswrittenm/s2).Forexample,ifanobjectgain6ms-1ofvelocityin2s,itsaverageaccelerationis3ms-2.Accelerationisavector.Theaccelerationvectoraboveisforaparticlewithanaccelerationof3ms-2totheright.However,aswithvelocity,itisoftenmoreconvenienttousea+or-forthevectordirection.,Acceleration,3ms-2,8,A1Motion,mass,andforces,Ifvelocityincreasesby3ms-1eveysecond,theaccelerationis+3ms-2.Ifitdecreasesby3ms-1eveysecond,theaccelerationis-3ms-2.Mathematically,anaccelerationof-3ms-2totherightisthesameasanaccelerationof+3ms-2totheleft.,Acceleration,9,A1Motion,mass,andforces,Onthevelocity-timegraphabove,youcanworkouttheaccelerationovereachsectionbyfindingthegradientoftheline.Thegradientiscalculatedlikethis:gainalongy-axisgradient=gainalongx-axis,Acceleration,timeins,velocityinms-1,acceleration=gradient=6/2=3ms-2,acceleration=gradient=0/2=0ms-2,acceleration=gradient=-6/2=-3ms-2,10,Forceisavector.TheSIisthenewton(N).Iftwoormoreforcesactonsomething,theircombinedeffectiscalledtheresultantforce.Twosimpleexamplesareshownbelow.Intheright-handexample,theresultantforceiszerobecausetheforcesarebalanced.Aresultantforceactingonamasscausesanacceleration.Theforce,mass,andaccelerationarelindedlikethis:resultantforce=massaccelerationF=maForexample,a1Nresultantforcegivesa1kgmassanaccelerationof1ms-2.(Thenewtonisdefinedinthisway.),A1Motion,mass,andforces,Force,11,A1Motion,mass,andforces,Themoremasssomethinghas,themoreforceisneededtoproduceanygivenacceleration.Whenbalancedforcesactonsomething,itsaccelerationiszero.Thismeansthatitiseitherstationaryormovingatasteadyvelocity(steadyspeedinastraightline).,Force,6N,6N,6N,6N,Resultantforce=12Ndownwards,Resultantforce=0,12,A1Motion,mass,andforces,OnEarth,everythingfeelsthedownwardforceofgravity.Thisgravitationalforceiscalledweight.Asforotherforces,itsSIunitisthenewton(N).NeartheEarthssurface,thegravitationalforceoneachkgisabout10N:thegravitationalfieldstrengthis10Nkg-1.Thisisrepresentedbythesymbolg.,Weightandg,mg,13,Inthediagramabove,allthemassesarefallingfreely(gravityistheonlyforceacting).FromF=ma,itfollowsthatallthemasseshavethesamedownwardacceleration,g.Thisistheaccelerationoffreefall.Youcanthinkofg:Titherasagravitationalfieldstrengthof10Nkg-1Orasanacceleratonoffreefallof10ms-2Inmoreaccuratecalculation,thevalueofgisnormallybakentobe9.81,ratherthan10.,A1Motion,mass,andforces,Force,14,Theturningeffectofforceiscalledamoment:momentofforce=forceperpendiculardistance*aboutapointfrompoint*measuredfromthelineofactionoftheforce.Thedumb-bellbelowbalancesatpointObecausethetwomomentsaboutOareequalbutopposite.,A1Motion,mass,andforces,Momentsandbalance,15,Thedumb-bellismadeupofsmallerparts,eachwithitsownweight.Together,theseareequivalenttoasingleforce,thetotalweight,actingthroughO.0isthecentreofgravityofthedumb-bell.,A1Motion,mass,andforces,Momentsandbalance,16,Thedensityofanobjectiscalculatedlikethis:TheSIunitofdensityisthekilogram/cubicmetre(kgm-3).Forexample,2000kgofwateroccupiesavolumeof2m3.50thedensityofwateris1000kgm-3.Densityvalues,inkgm-3alcohol800aluminium2700iron7900lead11300,A1Motion,mass,andforces,Density,17,Pressureiscalculatedlikethis:TheSIunitofpressureisthenewton/squareme