物理学专业英语A1

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 mass accelerationF 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 NeartheEarth ssurface 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 force perpendiculardistance 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 square