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1、achancetomakeatripintospacewithmyfriendLiYanping,anastronomer.Wevisitedthemooninourspaceship! Beforeweleave,LiYanpingexplainedtomethattheforeceofthegravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheair,wew
2、erepushedbackintoourseatsbecauseweweretryingtoescapethepulloftheearth’sgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.“Whyisthespaceshipnotfallingbacktotheearth?OntheearthifIfallfromatreeIwillfalltoground.”Iasked.“Wea
3、retoofarfromtheearthnowtofeelitspull,“heexplained,“sowefeelasifthereisnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth’s.“Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesma
4、llerandthemoonlarger. Whenwegotthere,Iwantedtoexploreimmediately.“Comeon,”Isaid.“Ifyouareright,mymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!”Ilaughedandclimbeddowntheste
5、psfromthespaceship.ButwhenItriedtostepforward,IfoundIwascarriedtwiceasfarasontheearthandfellover.“Ohdear,”Icried,“walkingdoesneedabitofpracticenowthatgravityhaschanged.”AfterawhileIgotthehangofitandwebegantoenjoyourselves. Leavingthemoon’sgravitywasnotaspainfulasleavingtheea
6、rth’s.Butreturningtotheearthwasveryfrightening.Wewatched,amazedasfirebrokeoutontheoutsideofthespaceshipastheearth’sgravityincreased.Againwewerepushedbackhardintoourseatsaswecamebacktoland.“Thatwasveryexhaustingbutveryexcitingtoo,”Isaid.“NowIknowmuchmoreaboutgravity!Doyouthin
7、kwecouldbisitsomestarsnexttime?”“Ofcourse,”hesmiled,“whichstarwouldyouliketogoto?” Caughtbyablackhole ThespaceshipwarnedLiYanpingandmetobereadyaswewereapproachingthe“blackhole”.Wecouldseesomefaintlightsaroundwhatlookedlikeanemptyspace.LiYanpingsaid,“Thatemptyholeislikeamouth
8、alwaysneedingtobefed.Thoselightsarethinsthatarebeingpulledintotimebythegrav
