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问题 计算题

额定功率为80KW的汽车, 汽车的质量m=２×１０㎏,在平直公路上行驶的最大速度为20m/s,如果汽车从静止开始做匀加速直线运动,加速度大小为2 m/s,运动过程中阻力不变,求:(1)汽车所受的恒定阻力。 (2)3s末汽车的瞬时功率。 (3)经过多长时间汽车功率达到额定功率。

答案

（1）fVm（2）4.8×104W（3）t2=5s

题目分析：(1)当速度最大时有P额=F牵V=fVm (1分)  ,f=N  (1分)

(2)匀加速过程有：F-f=ma   ，    F=f+ma=4×103+2×103×2=8×103N   (1分)

V1=at1=2×3="6m/s" (1分)    ,    P1=FV1=8×103×6=4.8×104W   (1分)

(3)当汽车刚达到额定功率时有 P额=FV2         ，V2=="10m/s" (1分)

V2=at2      (1分)                    ，得t2=5s   (1分)

点评：解决本题的关键知道发动机功率P=Fv，知道当速度达到最大时，牵引力等于阻力．以及知道以恒定加速度运动在整个过程中的运动情况．

阅读理解
阅读短文，完成表格。
Dear Ms White, 
       The office is looking for a student consultant (顾问) for the coming term. Could you help me find a person
for this position before February12, please?
       The consultant's job is to meet with students to discuss problems, opinions and suggestions on school
policies, present weekly spoken and written reports to the Student Affair Office, and attend different school
meetings as the student spokesperson (发言人). He should be a full-time student of our school and be skilled in
report writing and oral communication. It will be better if the applicant has worked as a student consultant
before.
       We will offer 2, 000 yuan a term for this position, and the time required for this job is 15 hours A week.
The e-mail address is: SAO-hr@sina.com.
       Thanks!                    
                                                                                                                         Mr. Green
Student Consultant of the Student Affair Office Wanted
Duties                     1.                                                         
Present weekly spoken and written reports
2.                                                         
Requirements3.                                                         
4.                                                         
Work experience as a student consultant perferred
Salary2, 000 yuan a term.
If you are interested in this position, please send your resume (简历) to SAO-hr@sina.com
before February 10.
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单项选择题

已知某500kV受端地区系统，由于电源不足需要从外区经较长的500kV线路输入有功功率约1000MW，请分析回答以下问题：

受端系统如果按输入有功功率相应配置的无功补偿设备，其安装地点应选择以下( )方案。

A．安装在500kV变电所高压侧；
B．安装在500kV变电所220kV侧；
C．安装在受端系统某个220kV变电所；
D．分别安装在由其受端系统供电的220kV及以下变电所中。
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