题目

如图所示，A、B为两块足够大的平行金属板，两板间距离为d，接在电压为U的电源上，在A板中央P点处放置一个电子放射源，可以向各个方向释放电子.设电子的质量为m、电荷量为q，射出的初速度为v，求电子打在B板区域的面积. 答案：解：打在最左边缘处的电子，将是类平抛运动的电子，在垂直电场方向做匀速运动，即r=vt.在平行电场方向做初速度为零的匀加速运动，即d=at2/2.其中a=，则t=d，将r=vt代入得r=vd.圆面积S=πr2=.【试题分析】There was a pilot(飞行员) and four people in a small plane.Suddenly there was something wrong with the machine(机器) while it was flying in the air. The smoke was everywhere in the plane.The pilot told the people there were only four parpchutes(降落伞).They all became worried and started to make excuses(借口). “I must go and mend the machine,”said the pilot , taking one of the parachutes.There was nothing he could do so he jumped out. The first person stood up. “I’m a doctor,”he said. “I help people live longer and I save lives”He also took a parachute and jumped out. The next person said , “I must have a parachute.I’m a very clever person.I have to attend (参加)an important sports match.I know I’ll win the game because I’ll be the cleverest person there.”He picked up a pack and jumped out. Two men were left--- an old businessman and a young mountain climber(登山者).By this time the plane was going down fast.The businessman said, “Young man,I’m old but you’re still young ,You take the last parachute.”The yougng mountain climber smiled , “Don’t worry.”he said, “We can both jump to safety because there are still two parachutes.Just now the clever person jumped out with my backpack.” 42.All the people became worried because________. 　 A.they couldn’t find the parachutes. B.they were afraid the plane would be broken and knew there were not enough parachutes for each person. 　 C.there was too much smoke and they couldn’t jump out from the plane. 　 D.they saw the pilot jump out first. 43.The clever man jumped out with_______. 　 A.a parachute　 B.nothing　　 C.the pack of the climber’s D. the businessman 44.All the people thought of themselves only except(除了)______。 　 A. the clever man　　　　　　 B. the businessman 　 C.the doctor　　　　　　　　 D. the young man 45.From the story we know that _____ would be dead（会死）. 　 A. the clever man　　　　　　 B. the businessman 　 C.the doctor　　　　　　　　 D. the young man