问题 选择题

对于万有引力定律的表达式,下列说法中正确的是(   )      

A.公式中G为引力常量,它是由实验测得的,而不是人为规定的

B.当r趋于零时,万有引力趋于无限大

C.两物体受到的引力总是大小相等的,而与m1、m2是否相等无关

D.两物体受到的引力总是大小相等、方向相反,是一对平衡力

答案

AC

题目分析:牛顿发现万有引力定律,对人们了解天体运动有较深的认识.一切物体均有引力,只不过有力的大小之分.

A、公式中G为引力常数,由卡文迪许通过实验测得.故A正确;

B、公式中从数学角度讲:当R趋近于零时其值是趋于无穷大,然而这是物理公式,所以R不可能为零.万有引力公式只适合于两个可以看做质点的物体,即,物体(原子)的自身半径相对两者的间距可以忽略时适用.而当距离无穷小时,相临的两个原子的半径远大于这个距离,它们不再适用万有引力公式.故B错误;

C、m1、m2之间的万有引力是属于相互作用力,所以总是大小相等,与m1、m2的质量是否相等无关,却与它们的质量乘积有关.故C正确;

D、m1、m2之间的万有引力总是大小相等方向相反,是一对相互作用力.不是一对平衡力.故D错误;

故选:C

点评:物理公式与数学表达式有所区别,物理公式中的一些量有一定的涵义.

选择题
完形填空
完形填空(共15小题, 每小题1分, 满分15分)
阅读下面的短文,从短文所给的A、B、C、D四个选项中,选出可以填入空白处的最佳答案。
For years we have been told that encouraging a child’s self-respect is important to his or her success in life.__41__ child experts are now learning that __42__ praise can lead to the opposite effect. Praise-aholic(受表扬上瘾的) kids who __43__ it at every turn may become teens who try to get the same kind of __44__ from friends when asked if they want to go in the backseat of the car.
The meaning of saying “You are the __45__ girl in class,” or talking about the goals she scored but not her general __46__, is that you love her __47__ when she looks the best, scores the highest, achieves the most. And this __48__ over to the classroom.
Social psychologist Carrol Dweck, PhD, tested the effects of overpraise on 400 fifth graders __49__ she was at Columbia University. She found that the kids who were __50__ for “trying hard” did better on tests and were more likely to take on difficult tasks than __51__ praised for being “smart”.
“Praising qualities or abilities leads to a false __52__ that success will come to you because you __53__ that quality, and it devalues(贬值) effort. __54__, children are afraid to take on challenges, ” says Dweck, now at Stanford University, “They think they’d better __55__while they’re ahead.”
小题1:  
A.And B.ButC.ThenD.So
小题2:  
A.enoughB.a little C.too muchD.some
小题3:  
A.expectB.rescueC.notice D.design
小题4:  
A.educationB.praise C.competitionD.direction
小题5:  
A.prettiestB.youngest C.tallest D.fattest
小题6:  
A.abilityB.valueC.kindnessD.effort
小题7:  
A.stillB.only C.already D.yet
小题8:  
A.carriesB.looksC.turnsD.takes
小题9:  
A.whileB.because C.until D.unless
小题10:  
A.set downB.looked through C.picked upD.thought highly of
小题11:
A.oneB.it C.that D.those
小题12:
A.idea B.plan C.schedule D.decision
小题13:  
A.haveB.likeC.adviseD.make
小题14:
A.After allB.At first C.As a resultD.In addition
小题15:  
A.move B.stopC.fight D.work