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Chapter4studyguide Workisdonewhenaforcecausesanobjecttomoveinthedirectionoftheforce. Example:acupofcoffeeissittingonthecounter(ithaspotentialenergy).Yourcatisajerkandknocksthecupoffthe counterandisinmotionfallingtowardsthefloor(kineticenergy). Aforcedoesnotalwaysresultinworkbeingdone. Example:youhelppushalargerockfromyourbackyardtowardsthewoods,buttherockdoesnotmove.Workhasnot beendoneeventhoughyouappliedforce. Theformulaforcalculatingtheamountofwork(W)doneinmovinganobjectrequiresyouto multiplytheforce(F)bythedistance(d)theobjectmoved.W=FXd. Forceisexpressedinnewtons(N).Metricisuniversallyusedinscience,andoneofthetoolsfor measuringdistanceisameterstick.Toexpresswork,usethenewton-meter(NXm). Joule:becauseworkisthetransferofenergytoanobject,thejoule(j)isalsoaunitusedtomeasure energy. Powerishowfastworkisdone.Tocalculatepower,divideworkbytime.P=w/t Theunitusedtoexpresspowerisjoulespersecond(J/s),alsocalledawatt.RecallyourPowerful Workoutactivity(stairclimbing).Completingthestairclimbfastorslowdidnoteffecthowmuchworkyou did,onlyhowquicklyyoudidthework. Machine:adevicethatmakesworkeasierbychangingthesizeordirectionofaforce.Amachine allowsforcetobeappliedoveragreaterdistance,whichmeansthatlessforcewillbeneededfor thesameamountofwork. Workinputistheworkdoneonamachine,theworkdonebythemachineisworkoutput.Work inputisalwaysgreaterthanworkoutputdueto(examples:friction,heatloss). Force-distancetrade-off:whenamachinechangesthesizeoftheforce,thedistancethrough whichtheforceisexertedmustalsochange.Forceordistancecanincrease,butbothcannot increaseatthesametime.Whenoneincreases,theothermustdecrease(thinkofthepulleyactivity whereyoudidthesamework,butwiththestringdoubledinlength,youdidthesameworkwitha greaterlengthofstring). Mechanicaladvantage:isthenumberoftimesthemachinemultipliesforce.Onepulleydidnot haveamechanicaladvantage.Twopulleyshadamechanicaladvantageof2.Mechanicaladvantage iscalculatedlikethis,mechanicaladvantage(MA=outputforce/inputforce). Mechanicalefficiency:isacomparisonofamachine’sworkoutputwiththeworkinput. Mechanicalefficiency=workoutput/workinputX100 Typesofmachines Levers(hammerpullinganail),pulleys(blockandtacklewith4ropesegmentshasamechanical advantageof4),wheelandaxle(turningafaucetonoroff),wedge(apairofinclinedplanes workingtogether{thinkofaknifecuttingawatermeloninhalf},screw(aninclinedplanewrapped inaspiralaroundacylinder).