Problem

“Wow,lookatalltheseterrificbooks!”youexclaimasyouskimoversomeofthetitlesinacardboard box.


“Yeah,”yourfriendagrees,“buthowarewegoingtogetthemtothe library?”


It’sBookDonationDayatschool.

Astrongstormhadsweptthroughtownamonthago.Partoftheschoolhadbeendamaged,includingthelibrary.Noonegothurt,buttherainandwindruinedmanyofthebooks.Sostudents,teachers,andparentsorganizedabookdrivetorestockthe library.


Whatasuccess!Peoplehavebeendroppingoffboxesofbooksallmorningatoneoftheschoolentrances.Heavyboxesofallsizesarestackedupandadozenvolunteersarereadytocarrythemtothe library.


Buttheboxesareheavy—reallyheavy.Thelibraryistoofarawaytopushordragtheboxes.Andit’shardforeventwopeopletocarryone.Youandtheothervolunteerscouldleavetheboxesandjustcarryafewbooksatatime,butthatwouldtaketoolong.Therehastobeabetter way.

Solution

Youthinkforamoment.“I’vegotit!Comeon,wehavetogototheMaintenance Office.”


Fiveminuteslater,youandyourfriendareallsmilesasyoucomewalkingdownthehallwaypullingtwodollycarts.Thevolunteerseasilystacktheboxesonthedollies.Thenit’seveneasiertorolltheboxestothelibrary,unloadthem,andheadbackformore.Witheveryoneworkingtogether—andwiththerighttools—thejobisdoneinnotime.Well done!

It's Simple

Whatmadethedolliessuchgoodtoolsformovingtheheavyboxes?It’ssimple!Thewheelsonthedollyarepartofasimplemachine.Likeallmachines,simplemachineshelpusdojobs,usuallybymovingthingsfaster,farther,ormoreeasilythanwecanwithoutthe machine.

axle

wheel

axle

Adollycartmaynotlooklikeatypicalmachine.It’snotbigandpowerfullikeacaroratrain.Itdoesn’tvroom!screech!orswoosh!likealawn mower,apowerdrill,oradishwasher.Theseareallcomplicatedmachineswithlotsofparts.Simplemachineshavenomorethanafewparts.Infact,somearemadeofonlyonepart.Nearlyeverymachine,nomatterhowcomplex,containsoneormoresimplemachines.Thinkaboutaschoolbus.Thehingesonwhichthedoorsswingopenarelevers.Thestairsyouclimbtoboardthebusareatypeofinclinedplane.Screwsmightholdtheframeoftheseatstothefloor.Andofcourse,thewheelsprovidethesmoothrideto school.

Youcanfindwheelsandaxlesonwagonsand bicycles.

MachinesThatGoRound

Ifamachineturnsinacircle,itisprobablyasimplemachinecalledawheelandaxle.Awheelisadiscthatturnsroundandround.Youprobablyknew that.


Wheelsdon’tworkbythemselves,though.Theyhavepartners—axles!Anaxleisarodconnectedtothecenterofawheel.Theaxleconnectsthewheeltoavehicle.Whentheaxlemoves,sodoesthewheel,andsodoesthe vehicle.

Wheelsandaxlesmakeiteasiertomovethings,likeawagonfulloflogsoraboxfullofbooks.Imaginepullingsuchaloadwithoutwheels.Thebottomofthewagonwouldscrapeagainsttheground.Asyoupullforward,theforceoffrictionpullsintheopposite direction.


Wheelsgiveyouabigadvantageinthistug‑of-war.There’slessrubbingandfrictionwithwheels.Also,wheelsroll.So,ittakesasmalleffortfromyoutopullthewagonandkeepit going.