精品文档_卫生类-相同项单选集1、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespeedrequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespeedhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestofthewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoairresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEarthbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspacetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredflightallthetime.WhatisthemostimportantfactorforarockettoescapefromtheEarth（）A.Totravelasfastasitcan.B.ToovercometheEarthgravity.C.Togetawayfromtheairresistance.D.Tohavestrongpowerinitsoperation.2、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespeedrequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespeedhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestofthewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoairresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEarthbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspacetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredflightallthetime.HowfastwillarocketgotoescapecompletelyfromtheEarthsgravity（）A.Seventymilespersecond.B.Seventhousandmilesperhour.C.Twenty-fivethousandmilespersecond.D.Twenty-fivethousandmilesperhour.3、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespeedrequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespeedhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestofthewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoairresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEarthbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspacetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredflightallthetime.AsthereisalwaystheEarthsgravitational(地心引力的)pull,people（）.A.acceptitwithoutquestioningitB.nevernoticeitspresenceC.hardlythinkabouthowtouseitD.oftentrytoescapefromit4、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespe