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1、achancetomakeatripintospacewithmyfriendLiYanping,anastronomer.Wevisitedthemooninourspaceship! Beforeweleave,LiYanpingexplainedtomethattheforeceofthegravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheair,wewerepushedbackintoour
2、seatsbecauseweweretryingtoescapethepulloftheearth’sgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.“Whyisthespaceshipnotfallingbacktotheearth?OntheearthifIfallfromatreeIwillfalltoground.”Iasked.“Wearetoofarfromtheearthnowtofeelitspull,“he
3、explained,“sowefeelasifthereisnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth’s.“Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger. Whenwegotthere,Iwantedtoexploreimmedia
4、tely.“Comeon,”Isaid.“Ifyouareright,mymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!”Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforward,IfoundIwascarriedtwiceasfarasonthe
5、earthandfellover.“Ohdear,”Icried,“walkingdoesneedabitofpracticenowthatgravityhaschanged.”AfterawhileIgotthehangofitandwebegantoenjoyourselves. Leavingthemoon’sgravitywasnotaspainfulasleavingtheearth’s.Butreturningtotheearthwasveryfrightening.Wewatched,amazedasfirebrokeoutontheoutsideofthespaces
6、hipastheearth’sgravityincreased.Againwewerepushedbackhardintoourseatsaswecamebacktoland.“Thatwasveryexhaustingbutveryexcitingtoo,”Isaid.“NowIknowmuchmoreaboutgravity!Doyouthinkwecouldbisitsomestarsnexttime?”“Ofcourse,”hesmiled,“whichstarwouldyouliketogoto?” Caughtbyablackhole Thespaceshipwarned
7、LiYanpingandmetobereadyaswewereapproachingthe“blackhole”.Wecouldseesomefaintlightsaroundwhatlookedlikeanemptyspace.LiYanpingsaid,“Thatemptyholeislikeamouthalwaysneedingtobefed.Thoselightsarethinsthatarebeingpulledintotimebythegrav
