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1、精品文档_卫生类-相同项单选集1、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundata
2、ll.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespee
3、drequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespeedhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestofthewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoa
4、irresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEarthbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspacetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredf
5、lightallthetime.WhatisthemostimportantfactorforarockettoescapefromtheEarth（）A.Totravelasfastasitcan.B.ToovercometheEarthgravity.C.Togetawayfromtheairresistance.D.Tohavestrongpowerinitsoperation.2、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissome
6、thingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecoul
7、dthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespeedrequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespee
8、dhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestofthewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoairresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEart
9、hbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspacetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredflightallthetime.HowfastwillarocketgotoescapecompletelyfromtheEarthsgravity（）A.Seventymilespersecond.B.Seventhousandmilesp
10、erhour.C.Twenty-fivethousandmilespersecond.D.Twenty-fivethousandmilesperhour.3、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowarditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperation
11、andmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownase
12、scapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespeedrequiredtoescapecompletelyfromtheEarthsgravityissevenmilespersecond,orovertwenty-fivethousandmilesperhour.Onceescapespeedhasbeenreachedbyaspacecraft(宇宙飞船),nofurtherpowerisneeded.ArocketaimedattheMoon,forinstance,will”coast”(滑行)therestoft
13、hewaybecausetheEarthsgravitycannotthenpullitback,andthereisnoairresistance(阻力)inspacetoslowitdown.This”coasting”isknownas”freefall”.ThatdoesnotmeantherocketisfallingdowntowardstheEarthbutthatitistravelingfreelyinspacewithouttheaidofpower,likeabicyclecoastingdownhill.Freefallisanimportantfeatureofspa
14、cetravel:itwouldbeimpossibletocarryenoughfueltoprovidepoweredflightallthetime.AsthereisalwaystheEarthsgravitational(地心引力的)pull,people（）.A.acceptitwithoutquestioningitB.nevernoticeitspresenceC.hardlythinkabouthowtouseitD.oftentrytoescapefromit4、EscapingfromtheEarthTheEarthhasaforcethatpullsthingstowa
15、rditself.Wecallthisforcegravity(地心引力).Thisissomethingwelivewithallthetime,andwetakeitforgrantedandhardlyeverthinkaboutit.Butitisamostimportantfactorinrocketoperationandmustbeovercomeifwearetogetanywhereinspace,oroffthegroundatall.Takethethrowingofaballasanexample.Thehardertheballisthrown,thefasterandhigheritwillgo.WhatisthesecretItsspeed.Ifwecouldthrowtheballhardenoughitwouldgoupandupforeverandnevercomedown.Thespeedatwhichitwouldhavetobethrowntodothisisknownasescapespeed.Ofcourse,wecannotthrowaballhardenoughbecausethespe
