H2 Physics Explanations

1 H2PhysicsExplanations Syllabusdocumenthttps://www.seab.gov.sg/docs/default-source/national-examinations/syllabus/alevel/2024syllabus/9749_y24_sy.pdf Measurement● Explainwhy…helpsimprovereliabilityReliabilityislinkedtopercentageuncertaintyMarks:- Givedefinition/formula:%uncertainty=absoluteuncertainty/datavalue(givengraph,thebestdatapointwouldbetheonewiththelargestdatavalue)- Largerdatavaluereduced%uncertainty ● Explainwhichquantityhasthegreatestcontributiontotheuncertaintyofsomevariable.Seeexpressionofvariable.“Quantityisraisedtothepowerofxxx,soitcontributesgreatesttotheuncertaintyofthevariable.” ● Howdoestakingmanyreadingsandtakingaveragehelptoreducerandomerror?- Absoluteuncertaintyisthesame- Percentageuncertaintyiscalculatedbydividingabsoluteuncertaintyoveralargermeasurement- %uncertaintyisreduced ● Sometimesqnaskutosuggestanothermethodtoincreaseaccuracyofmeasurement.Theyactuallymeanaccuracyasprecision:< Kinematics● Accelerationchangedfrom+to-inthesameinstant.Whyisthisnotpossible?2accelerationsinthesameinstant->2netforces->impossible ● Explainhowthepathtakenbytheparticlecanbeconsideredparabolic.Parabolicmotionconditions:1. Constantacceleration(likeinkinematics)a. Provethattheresultantforcehasconstantmagnitudeanddirection.Egweightandelectricforcehaveconstantmagnitudeanddirection,sothenetforcehasconstantmaganddir.2. Onecomponentofvelocityisindirofacceleration,oneperpendicularcomponentofvelocityhasnoacceleration. Forces● Previouscalculationsshowthattheobjectisnotinrotationalequilibrium.However, wheninuse,theobjectisinequilibrium.Explainwhy.Think:whatwasn'tconsideredinpreviouscalculations?Eg(weightofjib,counterjib,massesatbottom)

2 Thesecausemomentsaboutpivot→helpcauseobjecttobeinequilibrium ● Whataretheoriginsofupthrust 1. Upthrustoriginatesfromthepressuredifferencebetweentopandbottomofthebody.2. (whyistherepressuredifference)Aspressureincreaseswithincreasingdepth,bottomisatlargerdepth,soupwardforceonbottomishigherthandownwardforceontop3. Linkpressuretoforce:Upthrustisproductofpressuredifferenceandcrosssectionalarea. ● Showupthrust=value.Explainyourworking.Upthrust=weightoffluiddisplaced ● Giventhatthereare3forces,1stforceisvertical,2ndforcehasbothverticalandhorizontalcomponents,explainwhythe3rdforcecannotbeverticaliftheobjectisinequilibrium.2ndforcehase.g.leftwardshorizontalcomponentofforce.Fortheobjecttobeinequilibrium,netforce=0N.Hencethe3rdforceneedstohavearightwardshorizontalcomponent. Alternatively,sayvectorsummustbe0.Thendrawthetriangle. ● Whysmallobjectsincircularmotioncanbedangerousforsatellites “Dangerousforsatellites”<-collision<-force<-N2L<-momentum<-p=mv<-highvelocity ● Whyisspeedofmoleculevibrationlowerthanspeedofsoundingasspeedofsoundingasisspeedofenergytransferwhilespeedifvibrationisspeedofoscillation ● Originofenergytransferinsoundwaves?mechanicalenergyofvibratingobject ● Originofrestoringforceonamoleculeasitvibrates?Gashasweight,andcanexertpressure.Pressuredifferenceexertsrestoringforce. ● Originofforcethatacceleratestrolley,usingNewton'slaws?[3] Thinking:IneedtouseN2LandN3L.N2Lisforasinglebody,N3Lisfor2bodies.Identifythese2bodies.Trolley+cylindersystemandexertedgasfromthecylinder. 1. ORIGINofforceexertedongas:N2L>Gasincylinderhasrateofchangeinmomentum,soleftwardforceisexertedongas2. N3L:equalandopprightwardforceontrolley3. N2L:trolleyaccelerates

3 ● Suggestandexplaindirectionofreactionforce,giventhatthereistensionandweight.Horizontal:thereisahorizontalcomponentoftension.Weightonlyhasaverticalcomponent.Vertical:(*fromthepreviouspart,calculatedW,needtocalculateifTy =W).Sumofupwardtension…smallerthanweight,soreactionforcehasanupwardverticalcomponent. Dynamics● Whathappenstoforceonapropelledobjectwhenfluidisejectedout,andcompressedairoccupiesmorespace? Gasoccupymorespace->volincrease->pV=nRT->pressuredecrease->p=F/A->forcebyaironfluiddecrease->forceonpropelledobjectdecrease ● Explainhowmomentumisconservedinthecollisionofthespherewiththeceiling1) Identifythesystem:ceilingandsphere2) Keywords:equalandopposite3) Criticaldetails:directionofmomentumchangeSystemcomprisesofceilingandsphereUpwardgainofmomentumbyceilingisequalandoppositetodownwardlossofmomentumbysphereRightwardgainofmomentumbyceilingisequalandoppositetoleftwardlossofmomentumbysphereIflittlemarksnoneedtowriteaboutforces ● Whyisitnotpossibleforbothitemstostopatthesameinstant?Initialmomentumis…Ifbothstopatthesametime,momentumwillbezero.ThisviolatesPOCOM ● Givenav-tgraph,explainwhichcurvecorrespondstoabodywithsmallermass.Forces→momentumN3L→equalandoppositeforces→timeofcollisionisthesame→impulse=forcextime→equalandoppositechangeinmomentum→smallermass,solargerchangeinvelocity WEP● ByconsideringTRANSFORMATIONofenergy,explainthemotionofatruckmovingupaninclinewiththesameengineforce.⭐transformationoftypeofenergyintoanothertypeVStransferofenergy=workdoneChemicalE(notWDbyengine)→KE→GPE+ThermalE(notWDdoneagainstresistiveforces) ● ExplainwhetherworkdonebymotorispositiveornegativeMethod1)ConsiderdirectionsofforceanddisplacementMethod2)Conservationofenergy Circular Motion● Objecttiedtostringincircularmotion.Astheobjectundergoescircularmotion,thestringwrapsaroundapole.Explainwhythestringmaysnap.Formula:Fc=mv 2 /r

4 1m–Radiusdecreases,velocityincreases(bycommonsense)=>centripetalforceincreases1m–Stringbreakswhentensionexceedsbreakingpointofstring ● State(intermsofforces)conditionthat’snecessaryfor uniformCM(uniformCM=constantlinearvelocity,sonoforcethat’sparalleltolinearvelocity)Resultant forceisperpendicular tolinear velocityanddirectedtowardsthecentreof thecircular path. ● Objectmovesinacircularpathat constant speed.Explainwhyithasacceleration.Directionofvelocityconstantlychanges->velocitychanges->accelerationisrateofchangeofvelocity->thereisacceleration ● UseNewton’slawsofmotiontoexplainwhyabodymovingwithuniformspeedinacirclemustexperienceaforcetowardsthecentreofthecircle.[3]Qnanalysis:(1)uniformspeed+(2)circle.Separate.- *Mustexplaintheoriginofacceleration.Whichischangeinvelocity.- Forcetowardscentreofcircle←accelerationtowardscentreofcircle(N2L)←changeinvelocitytowardscentreofcircle←linearvelocityistangentialtocircularpath←changeinvelocityisperpendiculartothedirectionofitsmotion←uniformspeed- Needtoqualifybysaying“asthebodyhasuniformspeed,changeinvelocity…”.Basicallyuneedtostartwithquestion’sgivencause/scenario. ● StandardFcexplanation○ (Dir)Initially,Bisperpendiculartotheinitialdirectionofmotionofelectrons(notmotion).SoFbperpendiculartov.○ (Dir)ThisoccursthroughoutitstimeintheBfield○ (Mag)VandFbremainsconstantthroughout,souniformCM Remember: 💫timeelementfordirection+💫phrase“directionofmotion” ● ParabolicmotionAlsofocusondirectionandmagnitude:directionofforceisperpendiculartoinitialdirectionofmotionandisconstant,soobjectundergoesparabolicmotion. ● Proventhatanobjectwithinitialvelocityxwillnotbeincircularmotionatthetopofthecircle.Isthisconclusionthesameordifferentwithalargermass?Conclusionsame.Accelerationisindependentofmass. ● CarundergoingCMturnsright.Whydoesittilttotheleft?Protip:ifrllydkwhattowrite,justwritesmthprovidesFc [separatewheelfromcarbody]Wheel→frictionprovidesforrightwardFc Car→inertia,tendstocontinuemovingstraight ● EffectwhenangularspeedincAssumeradiusunchangedinchainofreasoning,althoughendgoaliseffectonradius:p ● ExplainwhywaterleaveswashingmachinethroughholesMustgeneraliseyourexplanationtoallholepositions

5 Atholes,thereisnoNCF.Radialcomponentoftheweightofwaterisinsufficienttoprovideforrequiredcentripetalforce. Oscillations● Stateandexplainfeaturesofthegraphshowingthatthemotionissimpleharmonic. Evidence:shapeofgraphsAccelerationanddisplacementareopposite:oneis(sine),otherisnegative(sine)Accelerationanddisplacementaredirectlyproportional:bothare(sine) - MUSTSTARTFROMGRAPH!!!Egnotstraightlinegraph→aandxnotdirectlyproportional ● ExplainhowtheexpressionshowsSHM.[3m][2m]definitionofSHM[1m]variables(representingω 2 )areconstants,so…isconstant ● F=-kx.HowdoesthisproveSHM?Fisproportionaltoacceleration,a.Henceaccelerationisproportionaltodisplacement.- Pastmistake:neverdefinea