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.

Mostpulleysareacombinationofmovablepulleysconnectedtooneormorefixedpulleys.Constructionsitesareterrificplacestoseepulleysinaction.Soareloadingdocks,wherecranesliftandlowercargoonandoff ships.

Youcanfindotherexamplesjustaboutanywheresomethingneedstoberaisedandlowered.Thatincludestheblindsinyourhomeorschool.Windowwashersusepulleystoraiseandlowertheirplatformhighabovecitystreets.Andwhatifahikeronamountainisstrandedandneedstoberescuedbyahelicoptercrew?Pulleysliftthehikerto safety.

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!