【Becausewecanfeelthatthingsareveryheavywethinkof“weight”asbeingafixedqualitybutitisnotreallyfixedatall.Ifyoucouldtakeaone-poundpacketofbutter4000milesoutfromtheearthitwouldweighonl】

　　Becausewecanfeelthatthingsareveryheavywethinkof“weight”asbeingafixedqualitybutitisnotreallyfixedatall.Ifyoucouldtakeaone-poundpacketofbutter4000milesoutfromtheearthitwouldweighonlyaquarterofapound.

　　Whywouldthingsweighonlyaquarterasmuchastheydoatthesurfaceoftheearthifwetookthem4000milesoutintospace?Thereasonisthis:allobjectshaveanaturalgravity(引力)forallotherobjects;thisiscalledgravitationalattraction(万有引力).Butthispowerofattractionbetweentwoobjectsgetsweakerastheyarefartherapart.Whenthebutterwasatthesurfaceoftheearthitwas4000milesfromthecenter.Whenwetookthebutter4000milesoutitwas8000milesfromthecenterwhichistwicethedistance.

　　Ifyoudoublethedistancebetweentwoobjectstheirgravitationalattractiondecreases“twotimestwo”.Ifyoumakethedistancethreetimesfarawayitgetsninetimesweaker(threetimesthree).Ifyoumakeitfourtimesfarawayitgetssixteentimesweaker(fourtimesfour)andsoon.

　　Sothisisoneofthefirstthingsweneedtoremember:theweightofanobjectinspaceisnotthesameasitsweightonthesurfaceoftheearth.

　　1.Whichofthefollowingisright?

　　A.Athinghasthesameweighteverywhereinspace.

　　B.“Weight”isafixedqualityinanobject.

　　C.“Weight”isnotaqualityinanobject.

　　D.Athing’sweightchangeswithitsplaceinspace.

　　2.Thereis between .

　　A.gravity;objects? B.power;allobjects

　　C.gravity;thetwoobjectsonly? D.power;twothings

　　3.A400poundman-madesatelliteissenttotheplace2000milesoutfromtheearth.Itsweightisabout .

　　A.200pounds B.ahalfpounds

　　C.100poundsD.300pounds

　　4.Ifwemakethedistancebetweenobjectsfivetimesasfaraway .

　　A.itgetstwenty-fivetimesweaker

　　B.theirweightgetstwenty-fivetimesweaker

　　C.theirgravitygetstwenty-fivetimesweaker

　　D.theirgravitygetstwenty-fivetimesstronger

　　5.Afterreadingthispassageweshouldrememberthatweknowtheusualweightofathing .

　　A.becauseitisonthesurfaceoftheearth

　　B.becauseitisweighed

　　C.becauseitisins