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1、achancetomakeatripintospacewithmyfriendLiYanping,anastronomer.Wevisitedthemooninourspaceship! Beforeweleave,LiYanpingexplainedtomethattheforeceofthegravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheair,wewerepushedback
2、intoourseatsbecauseweweretryingtoescapethepulloftheearth’sgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.“Whyisthespaceshipnotfallingbacktotheearth?OntheearthifIfallfromatreeIwillfalltoground.”Iasked.“Wearetoofarfromtheearthnowtof
3、eelitspull,“heexplained,“sowefeelasifthereisnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth’s.“Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger. Whenwegotthere,Iw
4、antedtoexploreimmediately.“Comeon,”Isaid.“Ifyouareright,mymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!”Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforward,Ifound
5、Iwascarriedtwiceasfarasontheearthandfellover.“Ohdear,”Icried,“walkingdoesneedabitofpracticenowthatgravityhaschanged.”AfterawhileIgotthehangofitandwebegantoenjoyourselves. Leavingthemoon’sgravitywasnotaspainfulasleavingtheearth’s.Butreturningtotheearthwasveryfrightening.Wewatched,amazedas
6、firebrokeoutontheoutsideofthespaceshipastheearth’sgravityincreased.Againwewerepushedbackhardintoourseatsaswecamebacktoland.“Thatwasveryexhaustingbutveryexcitingtoo,”Isaid.“NowIknowmuchmoreaboutgravity!Doyouthinkwecouldbisitsomestarsnexttime?”“Ofcourse,”hesmiled,“whichstarwouldyouliketogo
7、to?” Caughtbyablackhole ThespaceshipwarnedLiYanpingandmetobereadyaswewereapproachingthe“blackhole”.Wecouldseesomefaintlightsaroundwhatlookedlikeanemptyspace.LiYanpingsaid,“Thatemptyholeislikeamouthalwaysneedingtobefed.Thoselightsarethinsthatarebeingpulledintotimebythegrav
