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六级练习2016.01.30


       
       




(1)___________________________________.Some of the recent experiments make scientists wonder if an unknown force is at work in the universe.They have ascertained that there are four known forces in the universe,four forces that cause objects to move toward each other or,in some cases, away from each other.(2)___________________________________.The second is the "electromagnetic force",which holds atoms and molecules together and which,within the atom, holds the electrons to the central nuclei.The third is the "strong force",which holds the particles inside the central atomic nucleus together.The fourth is the "weak force",which allows some atomic nuclei to break down,producing radioactivity, and which causes the sun to shine.All the four forces are absolutely essential to the universe as we know it.Without them, matter could not exist,stars and planets could not exist, human beings could not exist.However, there is a problem about the existence of a fifth force.Until recently, scientists were quite convinced that there wasn't.(3)_______________________________.



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In order to explore universe,scentists have done quite a few experiments.
The first is the gravitation of force which hold you to the ground and will make you fall if you were not careful.
The four known forces appear to explain everything a fifth forcr seems unnecessary.

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