英语阅读给个标准答案Ifamotherpusheshersmallsoninaswing,givingonlyalightforceeachtimehereturns,eventuallyhewillbeswingingquitehigh.Thechildcandothisforhimselfbyusinghislegstoincreasethemotion,butboththe
英语阅读给个标准答案
Ifamotherpusheshersmallsoninaswing,givingonlyalightforceeachtimehereturns,eventuallyhewillbeswingingquitehigh.Thechildcandothisforhimselfbyusinghislegstoincreasethemotion,butboththemother'spushandthechild'slegmovementsmustoccuratthepropermoment,ortheextentoftheswingwillnotincrease.Inphysics,increasingtheswingisincreasingtheamplitude;thelengthoftheropeontheswingdeterminesitsnaturaloscillationperiod.Thisabilityofanobjecttomoveperiodicallyortovibratewhenstimulatedbyaforceoperatinginitsnaturalperiodiscalledresonance.
Resonanceisobservedmanytimeswithoutconsciouslythinkingaboutit;forexample,onemayfindanannoyingvibrationorshimmyinanautomobile,causedbyalooseenginemountvibratingwithincreasingamplitudebecauseofanout-of-roundtire.Thebulgeonthetireslapsthepavementwitheachrevolution;atthenaturalresonancepoint~oftheenginemount,itwillbegintovibrate.Suchvibrationscanresultinconsiderabledamageifallowedtopersist.Antherdestructiveexampleofresonanceistheshatteringofacrystalgobletbytheproductionofamusicaltoneatthenaturalresonantpointofthegoblet.Theenergyofthesoundwavescausesvibrationintheglass;asitsamplitudeincreases,themotionintheglassexceedstheelasticityofthegoblet,anditshatters.
Aninstrumentcalledatachometermakesuseoftheprincipleofresonance.Itconsistsofmanytinybars,looselyfastenedtogetherandarrangedsothateachbarcanslideindependentlyoftheothers.Movementofthebarscauseschangesinadial.Whenplacednexttoarotatingmotororengine,thetachometerpicksupslightvibrationswhicharetransferredtotheresonantbars.Thesebarsbegintomove,andtheresultingdialmaybereadtofindtherevolutionsperminuteofthemotorveryquickly.
74.Anobject,ifmovingrhythmicallywhenstimulatedinanaturalperiod,issaidto
A.vibrate.B.resonate.C.swing.D.oscillate.
75.Thedistanceaswingmovesfromitsrestingpositioniscalledits
A.revolution.B.movement.C.frequency.D.amplitude.
76.Atachometerisaninstrumentthatusesresonancetodetermine
A.thespeedofamotorinrevol