题目

如图所示，质量为5×10﹣8kg的带电微粒以v0=2m/s的速度从水平放置的金属板A、B的中央飞入板间，已知板长L=10cm，板间距离d=2cm．当UAB=1000V时，带电粒子恰好沿直线穿过板间．（取g=10m/s2） （1） 求两间电场强度的大小． （2） 当UAB为多大时粒子刚好擦上板边沿飞出？ 答案：解：当UAB=1000V时，电场强度为：E= Ud = 1000V0.02m=5×104V/m 解：当粒子擦上板边沿飞出时，粒子向上做类平抛运动，由题得知，偏转距离y= d2 ，水平位移x=L．由L=v0t，y= 12at2 ，得：a= dv02L2 = 2×10−2m×(2m/s)2(0.1m)2 =8m/s2，根据牛顿第二定律得：q UAB1d ﹣mg=ma，又mg=q UABd ，联立两式得：UAB1=1800VAs a youngster, there was nothing I liked better than Sunday afternoons at my grandfather’s farm in western Pennsylvania. Surrounded by miles of winding stone walls, the house and field provided endless hours of fun for a city kid like me. I was used to tidy living rooms that seemed to whisper, “Not to be touched!” 　　I can still remember one afternoon when I was eight years old. Since my first visit to the farm, I had wanted more than anything to be allowed to climb the stone walls surrounding the houses. My parents would never approve. The walls were old；some stones were missing, others loose and falling. Still, my idea to climb across those walls grew so strong that finally, one spring afternoon, I had all my courage to enter the living room, where the adults had gathered after Sunday dinner. 　　“I, uh—I want to climb the stone walls, ”I said. Everyone looked up. “Can I climb the stone walls?”Immediately voices of disagreement went up from the women in the room. “Heavens, no! You’ll hurt yourself!”I wasn’t too disappointed；the response was just as I’d expected. But before I could leave the room, I was stopped by my grandfather’s loud voice. “Now hold on just a minute, ”I heard him say. “Let the boy climb the stone walls. He has to learn to do things for himself. ” 　　“Go, ”he said to me, “and come and see me when you get back. ”For the next two and a half hours I climbed those old walls—and had the time of my life. Later I met with my grandfather to tell him about my adventures. I’ll never forget what he said. “Fred, ”he said, smiling, “You made this day a special day just by being yourself. Always remember, there’s only one person in this whole world like you, and I like you exactly as you are. ” 　　Many years have passed since then, and today I host the television program Mister Rogers’ Neighborhood, seen by millions of children throughout America. There have been changes over the years, but one thing remains the same：my message to children at the end of almost every visit. “There’s only one person in this whole world like you, ”the kids can count on hearing me say, “and people can like you exactly as you are.” 1．When the writer was small, he lived ________.   A．in the city　　　　　　　                   B．on the farm   C．with his grandparents　　　　　         D．away from his parents 2．The writer enjoyed his visits to the farm because ________.   A．there were old stone walls                    B．it was an exciting place for him   C．he liked his grandfather                        D．the living room there was clean 3．We can learn from the passage that the writer was ________.   A．adventurous         B．funny　　　　   C．smart　　           D．talkative 4．What did the writer learn from his grandfather and his own experience on the farm?   A．To do things for others.　　　　　      B．To do whatever he liked.   C．To be proud of himself.　　　　　      D．To be himself.