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