题干

滑板运动是一项陆地上的“冲浪运动”,滑板运动员可在不同的滑坡上滑行,做出各种动作给人以美的享受.如图甲所示,abcdef为同一竖直平面上依次平滑连接的滑行轨道,其中ab段水平,H=3m,bc段和cd段均为斜直轨道,倾角θ=37°,de段是一半径R=2.5m的四分之一圆弧轨道,O点为圆心,其正上方的d点为圆弧的最高点,滑板及运动员总质量m=60kg,运动员滑经d点时轨道对滑板支持力用Nd表示,忽略摩擦阻力和空气阻力,取g=10m/s2,sin37°=0.6,cos37°=0.8.除下述问(2)中运动员做缓冲动作以外,均可把滑板及运动员视为质点.

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       A man may usually be known by the books he reads as well as by the company he keeps.One should always live in the best company,whether it is of books or of men.

       A good book may be among the best of friends.It is the most patient and cheerful of companions.It always receives us with the same kindness,amusing and instructing us in youth, and comforting us in age.

       Men often discover their similarity to each other by the common love they have for a book just as two people sometimes discover a friend by the admiration which both entertain for a third.There is an old proverb,“Love me,love my dog.” But there is more wisdom in this:“Love me,love my book.” The book is a truer and higher bond (纽带) of union.Men can think,feel,and sympathize with each other through their favorite author.They_live_in_him_togetherand_he_in_them.

       A good book is often the best container of a life preserving the best that life could think out,for the world of a man’s life is,for the most part,the world of his thoughts.Thus the best books are treasures of good words and the golden thoughts,which become our steady companions and comforters.

       Books are by far the most lasting products of human effort.Temples and statues decay (腐朽),but good books survive.Time is of no account with great thoughts,which are as fresh today as when they first passed through their author’s minds ages ago.

     The great and good do not die,even in this world.Preserved in books,their spirits walked abroad.The book is a living voice.It is an intellect (智者) to which one still listens.