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1、CHAPTER6DynamicsI:MotionAlongaLineRecommendedclassdays:3BackgroundInformationChapter6isacontinuationofChapter5,withthefocusnowshiftingtoquantitativeproblemsolving.Learningtorecognizeandidentifyforcescarefullyandaccuratelyisoneofthelargesthurdlesstudentsface.Ifthey’vebeguntogetagoodhandl
2、eonthisfromChapter5,thenthepresentchapterbecomesmoreamatterofconsolidatingandusingthatinformation.Chapter5introducedfree-bodydiagrams,butlikelyyoudidn’tgettoomuchopportunitytohavestudentspracticedrawingfree-bodydiagramsinclass.Thatneedstobeanimportantfocusofthepresentchapter.Studentdiff
3、icultieswithfree-bodydiagramsinclude:•Identifyingaforcecorrectly,butnotknowingwhichwayitpoints.•Includingforcesexertedbytheobject,notjustforcesexertedontheobject.•Placingthetips,ratherthanthetails,ofthevectorsattheorigin,whichmakesithardtodeterminethex-andy-componentsoftheforces.ddd•Inc
4、luding“”maasaseparateforce.Afterall,Newton’ssecondlawsaysF=ma.Thiserrorislikelyacontinuedbeliefintheneedforsomekindof“forceofmotion.”•Notusingacoordinatesystem,orusinganinappropriateone.•Makingerrorsindeterminingthecomponentoftheforcevectors.Fortunately,thesearemostlytechnicalerrorsrath
5、erthanconceptualerrors.Mostcanbeovercomefairlyquicklywithfocusedpracticeandfeedbackondrawingfree-bodydiagrams.Aconceptualissuedoesarisewithmass,weight,andgravity.Dailylifemakesnorealdistinctionbetweenmassandweight,sostudentstendtousethetermsinterchangeably.Moreinterestingisthefactthatth
6、ere’snotanagreed-uponscientificdefinitionofweight.6-1Onecommondefinitionisthatweightisasynonymforthegravitationalforce.Althoughthisdefinitionseemsappealing,andwasusedinthefirsteditionofPhysicsforScientistsandEngineers,itsoonrunsintologicaldifficulties.Thegravitationalforcedoesn’talwaysc
7、orrespondtowhatascalereadsortoyoursensationofheaviness,andwe’releftneedingtoexplainawaythecommonideaof“weightlessness.”Manytextbooksjustifyallthiswithanadhocuseof“apparentweight,”butthenstudentsareleftjugglingtwodifferentconceptsofweight.Moresignificantly,weightcannotbejustanot
