问题 问答题

如图所示,小木箱ABCD的质量M=180g,高L=0.2m,其顶部挡板E的竖直距离h=0.8m,在木箱内放有一个质量为m=20g的小物块P(可视为质点).通过细轻绳对静止木箱施加一个竖直向上的恒力T,为使木箱能向上运动,并且当AD与挡板E相碰木箱停止运动后,P物体不会和木箱顶AD相碰,求拉力T的取值范围. (g=10m/s2)

答案

2N<T<2.5N

由牛顿第二定律得:

T-(M+m)g="(M+m)a                " ①

P与顶部恰好不相碰时,木箱与顶部相碰时的速度应满足:

vP2<2gL                           ②   

而vP2="2ah                         " ③   

联立①②③式得: 

代入已知数据得:T<2.5N       

要使木箱向上运动:T>(M+m)g="2N      "

所以T的取值范围为:2N<T<2.5N   

填空题
阅读理解

阅读理解

     In 1982 Steven Callahan was crossing the Atlantic alone in his sailboat when it struck something and

sank. He was out of the shipping lanes and floating in a life raft, alone. His supplies were few. His chances were small. Yet when three fishermen found him seventy-six days later, he was alive.

     His account of how he survived is fascinating. But the thing that caught my eye was how he managed

to keep himself going when all hope seemed lost. Giving up would have seemed the only wise option.

     When people survive these kinds of circumstances, they do something with their minds that gives them

the courage to keep going. Many people in similarly desperate circumstances give in or go mad.

     "I tell myself I can handle it," wrote Callahan. "Compared to what others have been through, I'm

fortunate. I tell myself these things over and over, building up fortitude(坚强意志)…."

     It struck me as something important. And I've told myself the same thing when my own goals seemed

far off or when my problems seemed too overwhelming. And every time I've said it, I have always come

back to my senses.

      The truth is, our circumstances are only bad compared to something better. But others have been

through much worse. So here, coming to us from the extreme edge of survival, are words that can give us

strength. Whatever you're going through, tell yourself you can handle it. Tell this to yourself over and over, and it will help you get through the rough spots with a little more fortitude.

1. The writer seemed more interested in ______.

A. something that made Steven Callahan want to cross the Atlantic

B. What made Steven Callahan live through the circumstances

C. what Steven Callahan thought of in the ban circumstances

D. what was wrong with his boat on the half way

2. From the third paragraph we can infer that ______.

A. the state of mind means one's life or death

B. keeping up one's spirit is especially easy for one

C. most people have good physiological quality

D. bad conditions can strengthen one's will

3. How did Steven Callahan build up his confidence in difficulty?

A. By comparing himself with others.

B. By trying over and over again to call help.

C. By encouraging himself once and again.

D. By trying out different ways of how to survive.

4. What did the writer get from the experience of Steven Callahan?

A .One shouldn't lose heart when in difficulty.

B. Encouragement comes from one's behaviors.

C. The strength of example is endless.

D. To experience the difficulty does good to one person.