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1、achancetomakeatripintospacewithmyfriendLiYanping,anastronomer.Wevisitedthemooninourspaceship! Beforeweleave,LiYanpingexplainedtomethattheforeceofthegravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheair,wewerepushedbackintoourseat
2、sbecauseweweretryingtoescapethepulloftheearth’sgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.“Whyisthespaceshipnotfallingbacktotheearth?OntheearthifIfallfromatreeIwillfalltoground.”Iasked.“Wearetoofarfromtheearthnowtofeelitspull,“heexplaine
3、d,“sowefeelasifthereisnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth’s.“Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger. Whenwegotthere,Iwantedtoexploreimmediately.“Comeon
4、,”Isaid.“Ifyouareright,mymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!”Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforward,IfoundIwascarriedtwiceasfarasontheearthandfellover
5、.“Ohdear,”Icried,“walkingdoesneedabitofpracticenowthatgravityhaschanged.”AfterawhileIgotthehangofitandwebegantoenjoyourselves. Leavingthemoon’sgravitywasnotaspainfulasleavingtheearth’s.Butreturningtotheearthwasveryfrightening.Wewatched,amazedasfirebrokeoutontheoutsideofthespaceshipastheearth’sgravi
6、tyincreased.Againwewerepushedbackhardintoourseatsaswecamebacktoland.“Thatwasveryexhaustingbutveryexcitingtoo,”Isaid.“NowIknowmuchmoreaboutgravity!Doyouthinkwecouldbisitsomestarsnexttime?”“Ofcourse,”hesmiled,“whichstarwouldyouliketogoto?” Caughtbyablackhole ThespaceshipwarnedLiYanpingandmetobereadya
7、swewereapproachingthe“blackhole”.Wecouldseesomefaintlightsaroundwhatlookedlikeanemptyspace.LiYanpingsaid,“Thatemptyholeislikeamouthalwaysneedingtobefed.Thoselightsarethinsthatarebeingpulledintotimebythegrav