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问题 计算题

质量为3×106kg的列车，在恒定的额定功率下，沿平直的轨道由静止出发，在运动过程中受到的阻力恒定，经1×103s后达到最大行驶速度72km/h.此时司机关闭发动机，列车继续滑行4km停下来．求：

(1)关闭发动机后列车加速度的大小；

(2)列车在行驶过程中所受阻力的大小；

(3)列车的额定功率；

(4)列车在加速过程中通过的距离．

答案

(1)0.05 m/s2　(2)1.5×105 N　(3)3×106 W　(4)16 km

题目分析：(1)关闭发动机后列车在阻力的作用下，滑行了一段距离后才停下来，列车做匀减速运动

a＝v2/(2s)＝202/(2×4000) m/s2＝0.05 m/s2.

(2)所受阻力Ff＝ma＝3×106×0.05 N＝1.5×105 N.

(3)达到最大速度时，牵引力等于阻力，则

P额＝Ffv＝1.5×105×20 W＝3×106 W.

(4)对加速过程应用动能定理P额t－Ffs1＝mv2/2，

代入数据解得s1＝16 km.

点评：本题中汽车以恒定功率启动，关键要灵活选择研究的过程，运用动能定理求解．

口语交际，情景问答题

按下列要求扩展语句（任选一题）

①请以“失败是成功之母”为结句，运用正反论证法加以拓展。

                                                                                   

②请以“失败是成功之母”为结句，运用假设推理法进行拓展。

                                                        

③请以“失败是成功之母”为首句，运用因果论证法加以拓展。

                                                                                         
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      I have been a student at Bentley College in Waltham for some time now, so I have a lot of experience and know how things work at Bentley. As a freshman(新生)at Bentley College you will be living in the

Tree Dorms, Slade Hall or Miller Hall. I would highly suggest that you choose to live in the Tree Dorms

because this is where most fresh-men are going to be living. There will be a lot of activities going on in the building and you will meet lots of people. Slade Hall is next to the parking lot and not far from the Tree

Dorms, so that would be your second choice. As Miller Hall is small and far away, it would be my last

choice for freshmen housing. If Bentley offers you the chance to live with second or third year students

you'd better refuse their offer and live with freshmen. Living with your classmates will make the transition (过渡) into college life a lot easier.

      As a freshman your classes should be easy. After freshman year your classes will become a lot more

difficult, so I advise that you get down to business early in the first year. There are two very easy things you can do to increase your knowledge as a freshman. The first is to just go to class and the second is to

always do your homework. I also highly advise that you do it on your own and try to stay disciplined. It is

tooeasy to put your homework away and then get really behind and not be able to learn everything before a mid-term or final exam.

1. The author advises freshmen to live in the Tree Dorms because ___________.

A. it is across the parking lot from Slade Hall

B. it is small and quite far away from the playground

C. there are many second and third year students there

D. there are many freshmen and more activities

2. Where is Slade Hall?                    

A. Near Miller Hall.          

B. Far from Tree Dorms.

C. Beside the car park.   

D. Near the classrooms

.

3. According to the author, how can freshmen improve knowledge?                      

A. Remember to do homework by discussing it with others.

B. Finish homework and go to class every day.

C. Try to spend more time in the library.

D. Be able to learn something before a mid-term or final exam.

4. The author writes the passage to ___________.

A. tell new students how to get used to college life

B. tell new students about their teachers and dormitory

C. show the differences between college and high school

D. tell readers about some funny things at Bentley College
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