lecture21_handout力和热-伊利诺伊香槟分校.ppt

Physics101 Lecture21Waves WavesOverview TypesSpeedTraveling harmonic SuperpositionStanding 05 TypesofWaves 8 SlinkyPreflight3 SupposethatalongitudinalwavemovesalongaSlinkyataspeedof5m s Doesonecoiloftheslinkymovethroughadistanceoffivemetersinonesecond 1 Yes2 No 12 VelocityofWavesAct 17 Aspringandslinkyareattachedandstretched Comparethespeedofthewavepulseintheslinkywiththespeedofthewavepulseinthespring A vslinky vspringB vslinky vspringC vslinky vspring Traveling harmonic Waves Wavelength Thedistance betweenidenticalpointsonthewave Amplitude ThemaximumdisplacementAofapointonthewave 20 y x t Acos wt kx AngularFrequencyw x y WaveNumberk Recall f v l PeriodandVelocity 22 f v l TravelingWavesExercise y x t Acos wt kx Labelaxisandticmarksifthegraphshowsasnapshotofthewavey x t 2cos 4t 2x atx 0 25 Preflight1 2 Supposeatravelingwavemovesthroughsomemedium Iftheperiodofthewaveisincreased whathappenstothewavelengthofthewaveassumingthespeedofthewaveremainsthesame 1 Thewavelengthincreases2 Thewavelengthremainsthesame3 Thewavelengthdecreases 26 Thewavelengthofmicrowavesgeneratedbyamicrowaveovenisabout3cm Atwhatfrequencydothesewavescausethewatermoleculesinyourburritotovibrate a 1GHz b 10GHz c 100GHz ACT 29 Recallthatv lf ACTSolution 30 Absorptioncoefficientofwaterasafunctionoffrequency 31 InterferenceandSuperposition Whentwowavesoverlap theamplitudesadd Constructive increasesamplitudeDestructive decreasesamplitude 34 ReflectionAct Aslinkyisconnectedtoawallatoneend Apulsetravelstotheright hitsthewallandisreflectedbacktotheleft ThereflectedwaveisA InvertedB Upright 37 StandingWavesFixedEndpoints Fundamentaln 1 2nodes ln 2L nfn v l nv 2L 44 StandingWaves f1 fundamentalfrequency lowestpossible 48 Aguitar sE stringhasalengthof65cmandisstretchedtoatensionof82N Ifitvibrateswithafundamentalfrequencyof329 63Hz whatisthemassofthestring Summary WaveTypesTransverse egpulseonstring water Longitudinal sound slinky Travelingy x t Acos wt kx orAsin wt kx Su