大学物理演示实验-英文版

BuildaRaftPoweredbySurfaceTension

Abstract

Haveyoueverwonderedwhyawaterstridercanwalkonwater?Orhowdetergentcancleanyourdishes?Ifyouobservecarefully,youcanfinddozensofsimilarlyinterestingphenomenathatarealllinkedtothesurfacetensionofwater.Thisscienceprojectwillhelpyouunderstandandmeasurethepropertiesofwatersurfacetension.

Objective

Inthisscienceproject,youwill:

Developanunderstandingofwatersurfacetension,

Understandchangesinwatersurfacetensionundervariousconditions,

Makemeasurementstoestimatethenetwatersurfacetension.

Introduction

Ifyou'veeverblownupaballoon,youknowit'sprettyeasytoblowupwithoutbecomingoutofbreathwhenit'smadeofsoft,stretchyrubber.That'sbecausetheballoonofferslittleresistancetobecomingstretchedoutasitisblownup.Butforaballoonmadeofthickerorstifferrubber,moreenergyisrequiredtoblowitup.Theballoonoffersmoreresistancetostretching.Thinkofaballoonasamodelforsurfacetension.How"stretchy"theballoonmaterialisdetermineshowmuchresistance(surfacetension)mustbeovercomebytheenergyofyourblowinginordertoinflatetheballoon.Surfacetensionisdefinedastheenergyrequiredtoincreasethesurfaceareabyaunitamount.

Liquidsalsoexperiencesurfacetension.Themolecules(smallindividualparticles)oftheliquidexperienceintermolecularattractions,whichsimplymeansthemoleculesarepullingandpushingawayfromeachother,justlikemagnetsbothattractandrepeleachother.Inthecaseofawatermoleculesurroundedonallsidesbyotherwatermolecules,everypullingforceisbalancedbyapushingforce.Thenet(whichmeansoverall)effectisnochange.Butatthesurface,whereairandwatermeet,thatisn'ttrue.Thewatermoleculesatthesurfaceexperiencemorepullsdownwardtowardtheotherwatermoleculesbelowthemthanupwardtowardtheair.Thisisthesurfacetensionofthewater.Youcanseeitatworkwhenyoufillaglass.Evenifthewaterisattherimoftheglass,youcanaddjustafewmoredropssothatthewaterisslightlytallerthantherim.Don'tbelieveit?Lookatthepicturebelowandthentryityourself!

Figure1.Becauseofsurfacetension,thetopofthisfullglassofwatercurvesoutward.Ifmorewaterisadded,itwilleventuallyspilloverthesideoftheglass.Notetheedgeoftheglass,andhowfarthewatercurvesoverit.

Inthisscienceproject,you'llmakeasmallraftfromaclearplasticsheet(atransparency),andseeifyoucanpropelitbytakingadvantageofthesurfacetensionofwater.

Terms,Concepts,andQuestionstoStartBackgroundResearch

Resistance

Surfacetension

Molecule

Velocity

Kineticenergy

Mass

Acceleration

Bibliography

Thesewebsitesaregoodintroductionstosurfacetension:

MassachusettsInstituteofTechnology.(n.d.).IntroductiontoSurfaceTension.RetrievedAugust12,2010,from

Exploratorium.(n.d.).StickyWater.RetrievedAugust12,2010,from

Thiswebsiteoffersanexplanationofvelocity:

Physics4Kids.(n.d.).Velocity,Speed,andMotion...OhMy!RetrievedAugust12,2010,from

Thiswebsiteexplainskineticenergy:

Zobel,E.A.(n.d.).KineticEnergy.RetrievedAugust12,2010,from

MaterialsandEquipment

Transparencies,likethoseusedonanoverheadprojector(1box);availableatyourlocalofficesupplystore

Marker

Ruler,metric

Scissors

Kitchensponges(1package)

Tape

Labnotebook

Liquidlaundrydetergent

Eyedropper;availableatyourlocalpharmacy

Stopwatch,accurateto0.01second(sec)

Scale,onethatcandetectsmallmassesingrams(g).Agoodkitchenscalewillwork.Alternatively,tryapostalscale.Youmightbeabletousetheoneatyourlocalpostoffice,oryourschoolmighthaveatriple-beambalance.

Liquidhandsoap

Toothpaste

Toothpicks(1box)

Water

Largebasinorsinktoholdthewaterandyourtransparencyraftinearlytrials

o Itshouldbelargeenoughthattheraftcantravelashortdistancewithinit.

o Makesureitcanbeeasilyemptied,asyouwillbefillingandre-fillingitwith

freshwateroften.

Bathtubforlatertrials

Volunteer

8-inch(in.)x10-in.pictureframe,plasticorwood(theframewillgetwet)

Disposableplasticcups,small(4)

Ziptiesortwistties

ExperimentalProcedure

ExploringtheExistenceofSurfaceTension

Thinkofashapeforyourraftanddrawtheshapeontothetransparency.

Theraftshouldbesymmetric.

Theraftshouldbesmallenoughthatitcantravelashortdistanceinyourbasinorsink,butstillbelargeenoughtoholdthespongepiece.

Youwillneedtocutaspaceinthebackoftheraftwherethespongewillbe.Itshouldbeslightlylargerthanthesizeyou'llbecuttingyoursponge(whichwillbeinstep3).

Thelengthoftheraftisquiteflexible,butwesuggestaninitialsizeof3-5cmlong,andawidththatisapproximatelyhalfofthelengthyouchoose.Recordthesizeinyourlabnotebook.

SeeFigure2foranexampleoftheraftdesign.

Figure2.Exampleofaraftdesign.Notethewayinwhichthesmallpieceofspongeisattachedtothetransparencytopreventitfrommovingandsinking.

Cutouttheraft.

Cutthespongeintosmall,identicallysizedpieces.Thespongeshouldfitintothespacecutinthebackoftheraft.

Runatoothpickhorizontallythroughoneofthesmallspongepiecessoenoughofthetwoendsofthetoothpickcanrestonthetransparency,thenattachthetoothpickendswithtapesothatthespongeisattachedtothesmallspaceatthebackoftheraft.SeeFigure2,above.Recordthesizeoftheraftandthespongeinyourlabnotebook.

Fillabasinorsinkwithtapwater.

Puttheraftontothewatersurfaceandletitfloat.

Usinganeyedropper,putadropofdetergentontothespongeattheendoftheraft.Ifonedropisn'tenough,putoneortwomore.Theraftdoesn'trequiremuchdetergenttostartmoving.

Observethemotion.Recordallobservationsinyourlabnotebook.

Repeatstep7withthesameraftandsponge,continuingtoadddetergentadropperfulatatime,inordertobecomefamiliarwiththeeffectsofthechangeinsurfacetensionthatdetergentcreates.

a.Note:Afterafewtrials,thewaterwillhavetoomuchdetergentinitforitssurfacetensiontochangeifmoreisadded.Whenthishappens,youwillneedtoreplaceyourwaterwithnewwater.Thiswillbefairlyoften.

Fromyourbackgroundresearch,youshouldknowthatdetergentsdecreasethesurfacetensionofwater.Howcanthishelptoexplainyourresults?

ExploringOtherSubstancesasSurface-Tension“Motors”forYourRaft

Fillupthebasinorsinkwithfreshwater.

Replacethespongeontheraftwithafreshsponge.

Puttheraftontothewatersurfaceandletitfloat.

Putadropofsoapontothespongeattheendoftheraft.Youcouldcarefullypumponedrop,orunscrewthepumplidandletonedropfallfromthetubethatisattachedtothepumpontothesponge.

Observethemotionandcompareittothefirstraft,whichuseddetergent.Recordyourobservationsinadatatable,liketheonebelow.

Substance

Used

DidtheRaftMove?Yes/No

FastestSpeed?

Yes/No

GoodorBadMotor?HowDidItAffectWater'sSurfaceTension?

Detergent(firstraft)

Toothpaste

Vegetableoil

Tablesalt

Repeatsteps1-5.Makesurethatyouusethesameraftforeverytest,andthatyoureplacethewaterwithnewwaterforeverydifferentsubstanceyoutest.Youcantryallsortsofsubstances:vegetableoil,cornstarch,salt,andevenvinegar!Tryoutanythingthatyouthinkmightmovetheraft.

a.Besureyouhavethreetrialsforeachsubstance.Thiswillensureyourresultsareaccurateandrepeatable.Recordallresultsinyourdatatable.

Whichsubstancesworkedasa"motor"?Whichonesdidn't?Whichsubstancewasthebestmotor?Canyourelateyourfindingsbacktosurfacetension?

ExploringtheEffectofShapeontheMotionofYourRaft

Nowtestdifferentraftshapes.Designanothershapeforyourraftanddrawtheshapeontothetransparency.

Youmightwanttotryanasymmetricshape.

Youmightwanttoexperimentwithdifferentsizes,ordifferentbasicshapes.

Cutouttheraft,leavingaspaceforasmallpieceofsponge,andinsertthespongeattheendoftheraft.Again,recordallmeasurementsinyourlabnotebook.

Preparefreshwaterinthebasinorsink.

Puttheraftontothewatersurfaceandletitfloat.

Putonedropofthesubstancethatworkedbestintheprevioussectionontothespongeattheendoftheraft.

Observethemotionandcompareittotheotherrafts.Usethesamesubstanceforyoursurface-tensionmotorforeachtrial.Recordyourobservationsinadatatable,suchastheonebelow.

a.Besureyouhavethreetrialsforeachsubstance.Thiswillensureyourresultsareaccurateandrepeatable.Recordallresultsinyourdatatable.

RaftShape

DidtheRaftMove?Yes/No

FastestSpeed?

Yes/No

WhyDidtheRaft'sShapeCauseThisMotion?

Largesquare

Small

square

Largecircle

Smallcircle

MeasuringNetSurfaceTension

PrepareanewraftwithashapesimilartotheoneinFigure3below.Youwillbetestingtheraftwithinan8-in.x10-in.pictureframe,sotheraftsizeissuggestedtobe3-5cmlong,andawidthapproximatelyhalfofthislength.

Figure3.Trymakingaboatwiththisarrowheadshape.Doesitperformbetterthanothershapesyouhavetested?

Weightheraftusingascale,ingrams.Thisistheboat'smass.

Youmightneedmoreroomforyourrafttotravelthistime,sofillabathtubwithtapwater.

Nowbuildachannel—abarriersittingatthesurfaceofthewater—thatwillonlyallowtheboattotravelinonedirection.Thepointofthechannelistohaveacontrolledpathfortheboattotravelalong,soyoucanaccuratelymeasurehowfaritgoes.Itwilltravelalongthe10-in.lengthofan8-in.x10-in.pictureframe.Decidewhichendwillbethelaunchingpoint,andmakemarksontheframethatare25cmawayfromthelaunchpoint.

Aperfectobjecttouseforthisisan8-in.x10-in.pictureframemadeofplasticorwood.Youonlywanttheframe,soremovetheglass,backing,etc.Next,tightly

connectfoursmall,disposableplasticcupsatthecornersoftheframeusingzipties(ortwistties,asinFigure4,below).Thesewillbeyouranchorsforyourchannel,becauseyouwantittoremainstationarywhiletherafttravelswithintheframe.Iftheframesimplyfloatedatthesurfaceofthewater,thenitcouldmovearound,especiallysinceyouwillbechangingthesurfacetension.

Figure4.Useapictureframeanchoredbyfilledcupstokeepthechannelstationaryinyourbathtub.

Placethechannelinyourbathtubandfillitsothattheframeisatthesurfaceofthewater.Tokeepthechannelanchored,filleachcupwithwater.

SettheraftinmotionusingthesubstancethatpropelledyourraftsthebestfromtheExploringOtherSubstancesasSurface-Tension"Motors"forYourRaftsectionandusearulerandastopwatchtomeasurethetimeittakesfortherafttotravelabout25cm.Youmightneedextrahelpforthefollowingsteps,soaskavolunteertoholdtherulerwhileyouwatchtheboatandrunthestopwatch.

Puttheraftatoneendofthechannelandputonedropofdetergentontothespongeattheendoftheraft.

Measurethetimerequiredfortherafttotravel25cminsidethechannel.

Calculatethevelocityandthekineticenergyoftheboat.TakealookattheBibliographysectiontostudyuponvelocityandkineticenergy.Theformulaforvelocityis:

Equation1:

V=Velocityd=Distance,incentimeters(cm)

t=Time,inseconds(s)

Nowyouneedtocalculatekineticenergy.Youwillneedyourmasstobeinkilograms(kg),soconvertyourgramsmeasurementtokilogramsbydividingby1,000(forexample:ifyourmassis2g,divideby1,000toget0.002kg).Youwillalsoneedyourvelocitytobeinmeterspersecond(m/s),soconvertyourcentimeters/secondtometers/secondbydividingby100(forexample:ifyouhave5cm/s,divideby100toget0.05m/s).

Equation2:

KE=(1/2)xmxV2

o KE=Kineticenergy,injoules(J)

o m=Mass,inkilograms(kg)

o V=Velocity,inmeters/second(m/s)

Repeatsteps