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Infall2006theNationalBasketballAssociation(NBA)startedusingbasketballsmadewithsynthetic(合成纤维)orman-madematerialinsteadofleather(皮革).Theymadethechangebecausetheywantedeverybasketballtheyusetofeelandbounce(弹起)thesame.Notallleatherballsareexactlyalikeinweightorhowtheybouncebutthesyntheticballsare.

Howeversomeplayerscomplainedrightawaythatthenewballsbounceddifferentlyandwereactuallyhardertocontrolthantheleatherones.

Physicshelpedanswerquestionsaboutwhythenewballsfeltdifferent．Forexample，thescientistsstudiedfriction(摩擦力)whichinthiscaseaffectstheabilityofaplayertoholdontoaball."Thegreaterthefrictionthebetteritwillbeinhishand"explainsascientist.

Testsonbothwetanddryballsshowedthatwhilethesyntheticballwaseasiertoholdontowhendryithadlessfrictionandbecamemuchhardertoholdontowhenwet．That'sbecausesweatstaysonthesurfaceofthesyntheticballsbutgetstakenintotheleatherballs.

Thescientistsalsotestedbounceandfoundthatthelogo(商标)printedonthenewballsmadetheirsurfaceuneven(不平)andcausedthemtobouncealittlestrangelycomparedwiththeleatherballs.

InJanuary，theNBAwentbacktousingtheleatherballs．Theyaren'tperfectbutfornowthat'sjustthewaytheballbounces．

( )1.WhywerethesyntheticballsexpectedtobeusedinNBAaccordingtothetext?

A.Becausetheyareeasiertocontrol． B.Becausetheywastelessmaterial．

C.Becausetheyfeelandbouncealike． D.Becausetheylookexactlyalike．

( )2.Theballiseasiertoholdonto_________

A.whenfrictionisless B.whenfrictionisgreater

C.whenitisheavier D.whenitbouncesstrangely

( )3.Thesyntheticballbouncesdifferentlyfromtheleatheronemainlybecause_________

A.itiswet B.there'salogoonit

C.itisdry D.itsweightremainsthesame

( )4.Theunderlinedwordsweatrefersto____________.

A.灰尘