Pulleyson the Move
Thepulleyatthetopofaflagpolestaysinoneplace.It’sattached,orfixed,tothepole,soit’scalledafixedpulley.Fixedpulleysarealsousedtoraisesailsonsomesailboats.Thepulleyisattachedtothetopofthemast.All asailorhastodotoraisethesailispulldownona rope.
Afixedpulleyisfineforliftingsomethingthatdoesn’tweighverymuch,likeabucketwithlunchinsideoraflag.Butwhatiftheloadismuchheavier?Forexample,mechanicssometimeshavetoliftaheavyengineoutofacar.Onepersoncouldneverliftitusingonefixed pulley.
So,what’stheanswer?Morepulleys!Youcanaddasecondpulleybelowthefixedone.Noticethatthissecondpulleyattachestotheload.Thatway,thepulleymoveswiththe load.
Addingthemovablepulleydoessomethingspecial:Itdoublesyourforce!It’slikehavingtwopeoplepullinginsteadofone.Butwhystopthere?Addingathirdpulleymultipliesyourforcebythree.Afourthpulley. . . well,yougetthe idea.
Pulleymultiplicationmayseemlikemagic,butit’ssciencethroughandthrough.Theropesupportstheloadbyloopingbeneaththepulleysthatliftit.Thissupporthelpsthe lifting.
Countthe segmentsofropeinthediagram.Thenumberofsegmentstellsyouhowmanytimesthatcombinationofpulleysmultipliestheforce.Countonlythesegmentsofropethatloopunderapulley,notthesegmentthatyoupullon.Thesegmentyoupullondoesn’tsupportthe load.
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Cranesliketheseusemanypulleystoliftheavy weights.
less
distance,
more
force
greater
distance,
less
force
Afixedpulleychangesthedirectionof force.
Asyouaddmorepulleys,youpulltheropeagreaterdistancebutuselessforceto move the load.
Mostpulleysareacombinationofmovablepulleysconnectedtooneormorefixedpulleys.Constructionsitesareterrificplacestoseepulleysinaction.Soareloadingdocks,wherecranesliftandlowercargoonandoff ships.
Youcanfindotherexamplesjustaboutanywheresomethingneedstoberaisedandlowered.Thatincludestheblindsinyourhomeorschool.Windowwashersusepulleystoraiseandlowertheirplatformhighabovecitystreets.Andwhatifahikeronamountainisstrandedandneedstoberescuedbyahelicoptercrew?Pulleysliftthehikerto safety.
Pulleysusedtogethermakelifting easy.
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Pulleysusedonsailboatsandshipsarecalled blocks.
blocks
Trade‑offs
Youknowthatthemorepulleysyouuse,themoretheymultiplyyourforce.Anotherwaytolookatitis:Themorepulleysyouuse,thelesseffortyouneedtoliftthesame load.
Imagineusingafixedpulleytoliftaheavyboxofbooksontothebackofatruck.Yougrabagoodholdontheropewithtwohandsandpull.It’sastrain,butfourtugslaterthebox isonthe truck.
Forthenextboxofbooks,youaddamovablepulleyconnectedtothebox.Yourforceisdoubled,whichmeansyoupulltheropewithonlyhalftheeffort.It’saloteasierbutyouhavetogiveeighttugsontherope.Doyouseethe pattern?
Eachtimeyouaddapulley,yousaveoneffortbutyouincreasethedistanceyouhavetopulltherope.That’sthethingaboutsimplemachines:Therearealwaystrade‑offs.Themorepulleysyouuse,theeasieritistopulltherope,butthefartheryouhavetopullit.Youtradedistanceforeffort.Youalsosaveyourselffromgettingasoreback,soit’sagood trade!
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block