如图所示，倾角为
的斜面处于竖直向下的匀强电场中，在斜面上某点以初速度
水平抛出一个质量为m的带正电小球，小球在电场中受到的电场力与小球所受的重力相等，地球表面重力加速度为g，设斜面足够长．问：小球经多长时间落到斜面?
t=（v0tanθ）/g
分析小球受力小球做类平抛运动
a=（qE+mg）/m="2g----------------- " ① 5分
小球落至斜面时：x="at------------------ " ② 2分
y=1/2at2---------------------------- ③ 2分
tanθ="y/x------------------------ " ④ 2分
由①②③④得t=（v0tanθ）/g-------------- ⑤ 5分
| 完形填空。 | |||
| Peter is seven years old. He's a student of Grade One. He 1 like school. It's Sunday today. The little boy doesn't 2 He's playing a ball. near a house, 3 grandma comes to him and begins to talk 4 his school with 5 "Who is the best student 6 your class,Peter?" She asks. "Of course I am."Peter answers. "Why?" "In my class only I 7 answer my teacher's question. " 8 question?" "He often asks us,'Who don't understand me?' only I put up my 9 ." "And who is the worst (最差的) in your class?" "Our, 10 is. " "Why?" "He can't answer even the easiest (最简单的) questions," peter says, "he always asks my classmates to answer them!" | |||
| ( )1. A. doesn't ( )2. A. go home ( )3. A. Her ( )4. A. With ( )5. A. he ( )6. A. at ( )7. A. can't ( )8. A. What ( )9. A. hand ( )10. A. classmates | B. don't B. go to school B. his B. on B. her B. can B. can B. Which B. hands B. friend | C. isn't C. go to work C. He C. in C. him C. in C. may C. how C. head C. students | D. aren't D. go to play a ball D. she D. about D. his D. of D. must D. Why D. foot D. teacher |