作业详细解答NO.4
《大学物理I》作业-No.03 角动量与角动量守恒-A-参考答案

《大学物理I 》作业 No.03 角动量 角动量守恒定律 (A 卷)班级 ________ 学号 ________ 姓名 _________ 成绩 _______一、选择题[ ]1、一质点沿直线做匀速率运动时,(A) 其动量一定守恒,角动量一定为零。
(B) 其动量一定守恒,角动量不一定为零。
(C) 其动量不一定守恒,角动量一定为零。
(D) 其动量不一定守恒,角动量不一定为零。
答案:B答案解析:质点作匀速直线运动,很显然运动过程中其速度不变,动量不变,即动量守恒;根据角动量的定义v m r L⨯=,质点的角动量因参考点(轴)而异。
本题中,只要参考点(轴)位于质点运动轨迹上,质点对其的角动量即为零,其余位置均不会为零。
故(B)是正确答案。
[ ]2. 两个均质圆盘A 和B 密度分别为A ρ和B ρ,若A ρ>B ρ,两圆盘质量与厚度相同,如两盘对通过盘心且垂直于盘面的轴的转动惯量各为A J 和B J ,则 (A) A J >B J(B) B J >A J(C) A J =B J(D) A J 、B J 哪个大,不能确定答案:B答案解析:设A 、B 联盘厚度为d ,半径分别为A R 和B R ,由题意,二者质量相等,即B B A A d R d R ρπρπ22=因为B A ρρ>,所以22B A R R <,由转动惯量221mR J =,则B A J J <。
[ ]3.对于绕定轴转动的刚体,如果它的角速度很大,则 (A) 作用在刚体上的力一定很大 (B) 作用在刚体上的外力矩一定很大(C) 作用在刚体上的力和力矩都很大 (D) 难以判断外力和力矩的大小答案:D 答案解析:由刚体质心运动定律和刚体定轴转动定律知:物体所受的合外力和合外力矩只影响物体运动的加速度和角加速度,因此无法通过刚体运动的角速度来判断外力矩的大小,正如无法通过速度来判断物体所受外力的大小一样。
作业解答第二版

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3 F2<0
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偏差计算`
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F1=F0–2X0+1 =–11 F2=F1+2Y1+1 =–10 F3=F2+2Y2+1 =-7
F4=F3+ Y3+1 =–2
坐标计算
X0=6 Y0=0 X1=X0–1=5 Y1=0
X2=5 Y2=Y1+1=1
X3=5 Y3=Y2+1=2
答:(1) 第一象限直线插补程序 设内存中三个单元XE、YE、FM分别存放直线终点横坐标、 终点纵坐标、加工点偏差。
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data ends
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修改通道号和计数器 修改采样值缓冲区指针
北京中医药大学远程教育《英语1》作业4答案

A型题:* 1. I like to get up early so that I can get plenty of work _____before lunch.[ 1分]A.doneB.being doneC.to doD.to be done* 2. If only I ______ the English exam without much hard work.[ 1分]A.will passB.passedC.could passD.can pass* 3. Identify the errors in the following sentence:He always acts as if he is the chairman although he is nothing at all.[ 1分]A.actsB.as ifC.isD.at all* 4. Identify the errors in the following sentence:Alice is the only one of those neighbors who are always willing to help the old.[ 1分]A.the onlyB.whoC.areD.the old* 5. The ______power of the people in this town has been decreasing because most young people have left for the big cities.[ 1分]A.shoppingB.purchasingC.enduringD.spending* 6. Identify the errors in the following sentence:"Excuse me," he said, with more dignity than I would thought of possible considering his appearance.[ 1分]A.thanB.thought ofC.consideringD.appearance* 7. Identify the errors in the following sentence:Bob has mistaken me as Joe.[ 1分]A.hasB.mistakenC.meD.as* 8. She ______ her things into luggage and was in great excitement for the holiday in the West.[ 1分]A.put inB.loadedC.burdenD.packed* 9. I've never heard ______ such a thing.[ 1分]A.fromB.ofC.outD.at* 10. Identify the errors in the following sentence:On his way to school yesterday morning Prof. Wang met an old friend and stopped talking with him for quite a while.[ 1分]A.On his wayB.metC.stopped talkingD.for quite a while* 11. Your remarks are rather ______ because every one stands by her.[ 1分]A.out of the placeB.in the placeC.out of placeD.in place of* 12. Identify the errors in the following sentence:Mary was shocked to realizing that it was time for tides to come in.[ 1分]A.was shockedB.realizingC.wasD.tides* 13. I received a ______parcel yesterday and I am still wondering who may have sent it.[ 1分]A.sincereB.dullplicatedD.mysterious* 14. The police investigation discovered that three young men were ______in the robbery.[ 1分]A.caughtB.involvedC.connectedD.tightened* 15. I don't like the way ______ you laughed at her.[ 1分]A.whyB.thatC.in thatD.at which* 16. There was a long ______ silence between them after his angry words.[ 1分]A.badB.awkwardC.quiteD.embarrass* 17. He said he really couldn't ______ to offend such people in those days.[ 1分]A.spendB.takeC.costD.afford* 18. The school bus was _____by a thick fog.[ 1分]A.held upB.broken offC.kept upD.put off* 19. Children are always longing ______ Christmas because they can be presented with a lot of gifts under the Christmas tree.[ 1分]A.ofB.intoC.forD.after* 20. The children at the kindergarten ______ from 1 to 100.[ 1分]A.counted onB.counted downC.counted outD.counted for* 21. It is the boy’s laziness that ______his failure in the exam.[ 1分]A.resulted fromB.brought inC.result inD.led to* 22. The Chairman of the Trade Union suggested ______ to look into the problem.[ 1分]A.to set up a special committeeB.setting up a special committeeC.having set up aspecial committee D.to set up a committee special* 23. I remember ______ the book to you the other day when I met you on my way to the library.[ 1分]A.returningB.to returnC.returnedD.being returned* 24. Poor old George has ______ the world since his business failed.[ 1分]e down ine down withe down toe down on* 25. I’m ______ for news about them.[ 1分]A.nervousB.anxiousC.eagerD.terrible* 26. Identify the errors in the following sentence:As soon as I showed her the trick, she bursted out laughing[ 1分]A.As soon asB.showed herC.bursted outughing* 27. Identify the errors in the following sentence:I got two vases in China. All of them are expensive.[ 1分]A.gotB.vasesC.AllD.are* 28. Vickie had most probably ______her for another sort of girl.[ 1分]A.regardedB.viewedC.mistakenD.looked* 29. I don't think ______ he believed my reason because he had turned a deaf ear to me.[ 1分]A.at every momentB.for momentC.for the momentD.for a moment* 30. Identify the errors in the following sentence:She is most eagerly to know where the treasure has been hidden.[ 1分]A.mosB.eagerlyC.the treasureD.has been hidden* 31. Identify the errors in the following sentence:When I got to the cinema, the film had already started; I ought to get there earlier.[ 1分]A.got toB.had already startedC.ought to getD.earlier* 32. Identify the errors in the following sentence:He wanted to know how the way they behaved.[ 1分]A.to knowB.ow the wayC.theyD.behaved* 33. ______ at the meeting that everyone present was convinced.[ 1分]A.So forcefully he spokeB.So forcefully did he speakC.So forcefully was hespoken D.So forcefully has he spoken* 34. Identify the errors in the following sentence:Not until my mother told me the sad news last night did I knew that our neighbor Dr. Li was killed in a car accident the week before.[ 1分]A.Not untilB.the sad newsC.did I knewD.was killed* 35. In order to ______ his face, he concealed his real name from us and pretended to be a noble man.[ 1分]A.bring backB.rescueC.handleD.save* 36. The space-shuttle will land on the 18th of this month, _____ the 25th.[ 1分]A.but forB.instead ofC.onD.in* 37. When he heard the news that he was selected to study abroad, he jumped _____ joy.[ 1分]A.withB.ofC.intoD.because of* 38. Identify the errors in the following sentence:Since the beginning of this century, more and more scientists have become interested in the way how the human brain works.[ 1分]A.Since the beginningB.have becomeC.inD.the way how* 39. Identify the errors in the following sentence:After criticizing our proposal in every detail, the director went on telling us that there was nothing personal in his comments.[ 1分]A.After criticizingB.in every detailC.telling usD.nothing personal* 40. The negotiation has already been broken ______.[ 1分]A.outB.awayC.offD.in* 41. He held himself ______ for fear of saying something he would regret.[ 1分]A.onB.aboutC.upD.in* 42. He felt rather______as he was the only person who wore sportswear at the dinner party.[ 1分]A.in placeB.in the wayC.by the wayD.out of the place* 43. I hired a boat ______ I could go fishing.[ 1分]A.andB.toC.so thatD.in order* 44. The newly-retired old man looked ______ at the article on display before he made a decision to make a purchase.[ 1分]A.closelyB.closeC.nearlyD.near* 45. “We’ll do what we can to get the goods ____on time,” said the manger of the company.[ 1分]A.reached fromB.deliveredC.returnede* 46. Identify the errors in the following sentence:No matter whatever he wanted, she would give it to him.[ 1分]A.No matterB.whateverC.wantedD.would* 47. We must begin testing this instrument, ______ difficult it is.[ 1分]A.no matter whatB.no matter howC.no matter soD.no matter however* 48. Only guests of the hotel enjoy the ______of using the private beach.[ 1分]A.privilegeB.possibilityC.favorD.advantage* 49. How we______ a chance to visit your great country![ 1分]A.looked forB.longed forC.waited forD.went for* 50. What the ______ is that thing on your head?[ 1分]A.hellB.homeC.earthD.hall* 51. Identify the errors in the following sentence:That is troubling me is that I don’t have much experience in international commerce.[ 1分]A.ThatB.isC.much experienceD.in international commerce* 52. He mistook me ______ my sister from my receding figure in darkness.[ 1分]A.forB.ofC.byD.into* 53. Jim ______ to act on his own judgment, but he didn't because as a soldier he had to obey the order.[ 1分]A.would preferB.would have preferredC.preferredD.might prefer* 54. The boy leaning against the wall ______ and ______ but______ nothing and finally burst into crying.[ 1分]A.heard ... hear, listened toB.listened ... listened, heardC.heard ... from,listened D.heard ... heard of, listened* 55. His mother helped him select the pop music ______ his taste as his birthday gift.[ 1分]A.up toB.inC.toD.off* 56. Identify the errors in the following sentence:If one does not work hard, you cannot expect to succeed.[ 1分]A.IfB.oneC.youD.to succeed* 57. Identify the errors in the following sentence:After making observations for a week, Sam found why Mr. Hob bs’s short factory was so inefficient.[ 1分]A.making observationsB.for a weekC.foundD.so* 58. Identify the errors in the following sentence:There are many faults in his homework.[ 1分]A.There areB.manyC.faultsD.homework* 59. Identify the errors in the following sentence:Jonnie was not barely able to walk without help because of the bad injury in his leg.[ 1分]A.was notB.without helpC.because ofD.bad* 60. The general designer was talking to the builders of the Grand Hotel while pointing ______ the map.[ 1分]A.forB.outC.dowuD.at。
“三违”详细清单

“三违”详细清单“三违”是指“违章指挥、违规作业和违反劳动纪律”的简称。
违章指挥:主要是指生产经营单位的生产经营管理人员违反安全生产方针、政策、法律、条例、规程、制度和有关规定指挥生产的行为。
违章指挥具体包括:生产经营管理人员不遵守安全生产规程、制度和安全技术措施或擅自变更安全工艺和操作程序,指挥者使用未经安全培训的劳动者或无专门资质认证的人员;生产经营管理人员指挥工人在安全防护设施或设备有缺陷、隐患未解决的条件下冒险作业;生产经营管理人员发现违章不制止等。
违章作业:主要是指工人违反劳动生产岗位的安全规章和制度(如安全生产责任制、安全操作规程、工作交接制度等)的作业行为。
违章作业具体包括:不正确使用个人劳动保护用品、不遵守工作场所的安全操作规程和不执行安全生产指令。
违反劳动纪律:主要是指工人违反生产经营单位的劳动纪律的行为。
违反劳动纪律具体包括:不履行劳动合同及违约承担的责任,不遵守考勤与休假纪律、生产与工作纪律、奖惩制度及其他纪律等。
“三违”行为具体分类如下:NO.1 作业性违章1、上岗未出示安全作业证、未佩戴工作证。
未经三级安全教育上岗作业。
2、厂区内吸烟。
3、酒后上岗。
4、非特种作业人员从事特种作业。
4.1、未取得特种作业证的员工从事压力容器、起重、液氯充装、气割、电焊等特种作业。
4.2司机无特种车辆作业证开叉车、铲车。
5、检修设备时安全措施不落实,开始检修。
5.1、清洗置换不合格进行检修作业5.2、与系统未断开进行检修作业5.3、在未泄压的情况下进行拆卸法兰、阀门。
5.4、设备管线未置换、置换、吹扫。
6、停机检修后的设备,未经彻底检查启用。
7、进行检维修作业时,未关闭设备电源、水源及其它能源供应,释放残余能量,监护措施未落实及未设置警戒区域或未挂警示牌。
7.1、人员随意穿越警戒区域或警戒线。
8、发现隐患,未排除、报告,冒险作业。
9、工作前未检查设备(施)或设备(施)带故障、安全装置不齐全便进行操作。
九上化学4习题答案

九上化学4习题答案九上化学4习题答案在学习化学的过程中,习题是巩固知识和检验理解的重要工具。
本文将为大家提供九上化学第四章的习题答案,帮助大家更好地掌握相关知识。
第一题:下面是几种常见的化学物质,请根据其化学式写出它们的名称。
1. H2O:水2. CO2:二氧化碳3. NaCl:氯化钠4. CH4:甲烷5. C6H12O6:葡萄糖第二题:下面是几个物质的化学式,请根据其名称写出化学式。
1. 水:H2O2. 二氧化碳:CO23. 氯化钠:NaCl4. 甲烷:CH45. 葡萄糖:C6H12O6第三题:下面是几个物质的化学式,请判断它们是离子化合物还是分子化合物。
1. H2O:分子化合物2. CO2:分子化合物3. NaCl:离子化合物4. CH4:分子化合物5. C6H12O6:分子化合物第四题:下面是几个物质的化学式,请根据它们的化学式判断它们是酸、碱还是盐。
1. H2SO4:酸2. NaOH:碱3. NaCl:盐4. HCl:酸5. KOH:碱第五题:下面是几个物质的化学式,请根据它们的化学式判断它们是有机物还是无机物。
1. CO2:无机物2. CH4:有机物3. H2O:无机物4. C6H12O6:有机物5. NaCl:无机物第六题:下面是几个物质的化学式,请根据它们的化学式判断它们是单质还是化合物。
1. O2:单质2. CO2:化合物3. H2O:化合物4. N2:单质5. HCl:化合物第七题:下面是几个物质的化学式,请根据它们的化学式判断它们是金属还是非金属。
1. Na:金属2. H2O:非金属3. Fe:金属4. CO2:非金属5. Ca:金属第八题:下面是几个物质的化学式,请根据它们的化学式判断它们是酸性物质、碱性物质还是中性物质。
1. H2SO4:酸性物质2. NaOH:碱性物质3. HCl:酸性物质4. KOH:碱性物质5. H2O:中性物质通过以上习题的练习,相信大家对九上化学第四章的知识有了更深入的理解。
《大学物理AII》作业 No.04 光的偏振(参考答案)

放置。
I I 0 cos 2 cos 2 (90 ) I 0 cos 2 sin 2
0 当 45 时,出射光强最大, I max
1 I 0 sin 2 (2 ) 4
1 I0 4
*3、在夏天,炽热的阳光照射柏油马路发出刺眼的反光,汽车司机需要戴上一副 墨镜来遮挡。是否可用偏振片做眼镜?这比墨镜有什么优点? 答: 一般来讲,从路面上发出的直接射入人眼的反射光要比来自其他目标的 漫射光强烈得多。 通常把这种刺眼的强光叫眩光。眩光对司机的安全行车有严重 的影响。戴上墨镜,可以减弱射入眼睛的光强,但并不能限制眩光。如果用偏振 片做眼镜, 情况就大不相同了。 因为阳光 (自然光) 经路面反射后是部分偏振光, 在入射角接近布儒斯特角时偏振度很高, 近似为线偏振光。 对于水平的路面来说, 反射部分偏振光的强度极大的分量总是在水平方向。因此,只要使眼镜上的偏振 片的偏振化方向为竖直方向, 就可以强烈地吸收刺目的眩光,而对来自其他目标 的漫射光的吸收则要相对小得多。这种有选择地吸收眩光的优点是墨镜所没有 的。 此外,墨镜对光的吸收往往对波长有选择作用,会改变观察目标的色调。而 偏振片对光的吸收只对偏振态有选择作用,对波长并无选择吸收,不会改变目标
n2 )时,反射光 n1
将为线偏振光,并且反射光与折射光的夹角为( )。(用 n1 和 n2 表示两种介 2
质的折射率) 6、单轴晶体内有一确定的方向,光沿该方向入射时折射光在晶体内只有一束, 该方向称为(光轴);除了这个方向外,一般情况下入射光在晶体内都将分为两 束,一束称为(寻常光)或者 O 光,一束称为(非常光)或者 e 光。在晶体内, O 光沿各个方向传播速度(相同),,因而 O 光的子波面为球面;e 光沿各个方 向传播速度(不同),因而 e 光的子波面为旋转椭球面。对于正晶体而言 O 光 速度(>)e 光速度,O 光折射率(<)e 光折射率(填>或<);对于负晶体而言, 以上情况正好相反。 7、发生双折射的晶体内,O 光和 e 光都为(线偏振光);O 光的振动方向(垂 直)于其主平面,e 光的振动方向(平行)于其主平面。当入射光在晶体主截面 内时, O 光和 e 光的主平面与主截面重合, 此时 O 光与 e 光振动方向相互 (垂直) 。 8、尼科耳棱镜是利用(双折射现象)定理和(全反射)原理做成的起偏器,同 时尼科耳棱镜也可做为检偏器使用。 一束入射光通过旋转的检偏器, 如果光强 (周 期性变化,并且每旋转一周出现两次最大,两次消光)则可判断入射光是线偏振 光;如果光强周期性变化,但没有消光现象则可判断入射光是(部分偏振光)或 者 (椭圆偏振光) ;如果光强没有变化, 则入射光是(自然光)或者(圆偏振光) 。 9、一束自然光和线偏振光的混合光,让它垂直通过一偏振片。若以此入射光束 为轴旋转偏振片, 测得透射光强度最大值是最小值的 5 倍,那么入射光束中自然 光与线偏振光的光强比值为(1:2)。 解:设入射自然光光强为 I1,线偏振光光强为 I2,混合光通过偏振片后光强为:
大学英语1在线作业4满分答案及考点分析---精品模板

1. A:Must we finish the work right now?B: No,you ________. You can do it tomorrow。
A needn’tB shouldn’tC mustn'tD can’t词汇与结构2013答案A考点:考查情态动词2。
This is George,______ class you will be taking。
A whoseB whomC whoD his词汇与结构2013答案A3. What is the train ___________ to Birmingham?A feeB tripC fareD cost词汇与结构2013答案C4。
The reporters had to fight each other for a chance to ______ the famous singer。
A instructB questionC interviewD invest答案C5.Selecting a mobile phone for personal use is no easy task because technolgy ___so rapidly.A is changingB has been changedC will have changedD will change词汇与结构2013答案A6。
For a ______ of reasons,our team will not be participating in the upcoming match.A differenceB varietyC communityD branch词汇与结构2013答案B7。
A woman came in, ______ by her daughter。
A followedB followingC followD being followed词汇与结构2013答案A8。
pep人教版六年级英语下册unit4《Then and now》单元作业练习题附答案 - 副本

Unit 4 Then and nowPart A 第一课时一、读一读,翻译下列单词。
1. easy ()2. before ()3. look up ()4. Internet ()5. ago ()6. change ()7. took ()8. at all ()9. star () 10. time ()二、读一读,选择正确的选项。
( ) 1. — Tell me about your new school, please.— ________________.A.The teachers’ office is on the second floor.B.There was no gym in my time.C.There is a new library in it.( ) 2. — Could you see stars at night?— ________________.A.Yes, I can.B.Yes, I could saw them on the hill.C.No, I could. I liked stars.三、阅读短文,判断正(T) 误(F)。
This was my school five years ago. There was no library or gym in my old school. There were no computers either. I was in Grade 1 and we had to have PE class in the classroom because there was no playground outside. Now our school has changed a lot. I love the new school.( ) 1. My school has changed a little.( ) 2. There were some computers in my school five years ago.( ) 3. I’m now in Grade Six.( ) 4. There was one small playground in my old school.Part A 第二课时一、补充单词。
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©西南交大物理系_2015_02《大学物理AI 》作业 No.04 机械能 机械能守恒定律班级 ________ 学号 ________ 姓名 _________ 成绩 _______一、判断题:(用“T ”和“F ”表示)[ F ] 1.一个不受外力作用的系统,它的动量和机械能都守恒。
解:不受外力作用的系统,0=∑外F r,动量是守恒的。
而机械能守恒的条件是:0=+∑∑非保内外A A ,不受外力作用的系统只能保证外力做功的代数和为0,不能保证非保守内力做功的代数和为0,所以机械能不一定守恒。
[ T ] 2.质点系的内力可以改变系统的总动能,不能改变系统的总动量。
解:质点系的内力的矢量和为0,所以不会改变系统的总动量,而质点系内力做功的代数和不一定为0,因而可以改变系统的总动能。
[ F ] 3.质点运动过程中,作用于质点的某力一直没有做功,表明该力对质点的运动没有产生任何影响。
解:比如单摆运动中绳子的拉力,时刻与位移垂直,不做功,但是它对摆球的运动当然有影响。
[ F ] 4.保守力作正功时,系统内相应的势能增加。
解:保守力做功数值上等于相关势能增量的负值。
所以当保守力作正功时,系统内相应的势能会减少。
[ F ] 5.图示为连接a 点和b 点的三条路径。
作用力F v对一质点做功,经由图示方向和路径,功的示数表示在图中。
由此可以判断F v是保守力。
解:保守力做功的特点:做功与路径无关。
二、选择题:1. 一个质点同时在几个力作用下的位移为)SI (654kj i r v v v v+−=Δ, 其中一个力为恒力)SI (756kj i Fv vv v+−=,则此力在该位移过程中所作的功为[ B ] J 76)A (J 19)B ( J 71)C (J 76)D (−解:由功的定义,F r力的功为(J)91422524)654()756(=++=+−⋅+−=Δ⋅=k j i k j i r F A r rr r r r v r2. 一质点在如图所示的坐标平面内作圆周运动,有一力)(0j y i x F F vv v +=作用在质点上。
在该质点从坐标原点运动到),(R R 位置过程中,力F v对它所作的功为[ A ] 20)A (R F202)B (R F203)C (R F 204)D (R F解:由功的定义,F r力的功为∫∫∫+=⋅=y F x F r F A y x d d d rr 20000d d R F y y F xx F R R =+=∫∫3.如图,一质量为m 的物体,位于质量可以忽略的直立弹簧正上方高度为h 处,该物体从静止开始落向弹簧,若弹簧的劲度系数为k ,不考虑空气阻力,则物体下降过程中可能获得的最大动能是 [ C ] (A) mgh(B) k gm mgh 222−(C) kg m mgh 222+ (D) kg m mgh 22+解:以m 、弹簧、地球所组成的系统作为研究对象,系统机械能守恒。
物体动能最大时,位于物体所受合外力为零的地方,即弹力等于重力的地方:kmgx kx mg =⇒=,以此位置作为重力势能0点,根据机械能守恒: 22max 2121)(kx mv x h mg +=+,将kmg x =代入得到 k g m mgh k mg k k mg h mg mv 221212222max +=⎟⎠⎞⎜⎝⎛−⎟⎠⎞⎜⎝⎛+=4.一个作直线运动的物体,其速度v 与时间t 的关系曲线如图所示。
设时刻1t 至2t 间外力作功为1W ;时刻2t 至3t 间外力作的功为2W ;时刻3t 至4t 间外力作功为3W , 则 [ C ] (A) 0,0,0321<<>W W W(B) 0,0,0321><>W W W(C) 0,0,0321><=W W W(D) 0,0,0321<<=WWW解:根据质点的动能定理 k E W Δ=v t t 间,~21不变, 0,01==ΔW E k 所以 减小v t t 间,~320 ,02<<ΔW E k增大v t t 间,~430,03>>ΔW E k5.如图所示,圆锥摆的小球在水平面内作匀速率圆周运动,判断下列说法中正确的是[ A ] (A)重力和绳子的张力对小球都不作功。
(B)重力和绳子的张力对小球都作功。
(C)重力对小球作功,绳子张力对小球不作功 (D)重力对小球不作功,绳子张力对小球作功。
答:(A)对。
无论是重力还是绳子张力与小球位移都时候重直,所以都不作功。
三、填空题:1.质量为kg 10的质点,在合外力作用下做曲线运动,该质点的速度为k i t v v v v 1642+=(SI) ,则在t = 1 s 到t = 2 s 时间内,合外力对该质点所做的功为 1200 J 。
解: 由k i t v v v v 1642+=:s t 1=()J v v m mv E k i v yx k 13602121164222111=+==⇒+=v v vst 2=()J v v m mv E k i v y x k 256021211616222222=+==⇒+=v v v根据动能定理:J E E E AK K K 120012=−=Δ=2.一块kg 10的砖头沿x 轴运动。
它的加速度与位置的关系如图所示。
在砖头从x =0运动至x =8.0m 的过程中,加速的力对它所做净功W = 800 J 。
解:由图可以得出:mx ma F x a2525==⇒=, ()J 800d 25d 24588080====⎟⎠⎞⎜⎝⎛∫∫mx x mx x F A3.一个质量为m 的质点,仅受到力3/r r k F rr =的作用,式中k 为常数,r r 为从某一定点到质点的矢径。
该质点在0r r =处被释放,由静止开始运动,则当它到达无穷远时的速率为 。
解:由功的定义,在该过程中变力F r作的功为200d d r k r r k r F A r r =⋅=⋅=∫∫∞∞r r根据质点的动能定理有:0212−=mv A即质点到达无穷远处的速率为22r m km A v ==4.一质量为m = 5 kg 的物体,在0到10秒内,受到如图所示的变力F 的作用,由静止开始沿x 轴正向运动,而力的方向始终为x 轴的正方向,则10秒内变力F 所做的功为 。
解:方法一:由F -t 图可知,物体各时段受力为⎩⎨⎧≤≤≤≤=)s 10s 5(20s)5s 0(8t x t F0 ~ 5 秒内应用动量定理:0d 855−=∫mvt t 得5秒末速度)s (m 20554125−⋅=×=v5 ~ 10 秒内再应用动量定理:51010d 20mv mv t −=∫得10秒末速度)s (m 4020205)510(201510−⋅=+=+−=v v 根据质点的动能定理,10秒内变力作的功为 (J)4000405210212210=××=−=mv A方法二:根据定积分的几何意义,F-t 曲线下的面积就等于物体从0-10S 内受到的冲量。
N.S20020540521d 10=×+××=∫==−tF St F I 根据动量定理:()112m.s 40−=⇒=−=Δ=v mv P P P I根据质点的动能定理,10秒内变力作的功为 (J)4000405210212210=××=−=mv A5.如图所示,质量为m 的小球系在劲度系数为k 的轻弹簧一端,弹簧的另一端固定在O 点。
初始时,弹簧在水平位置,原长为0l 处于自然状态。
小球由位置A 释放,下落到O 点正下方位置B 时,弹簧的长度变为l ,则小球到B 点时的速度大小为20)(2l l mkgl v B −−=。
解:以小球、弹簧和地球组成的系统为研究对象,系统在小球运动过程中只有保守内力----弹力作功,系统机械能守恒。
设B 点重力势能为零,弹簧原长弹性势能为零,则对于A 点,机械能为:l g m ,对于B 点,机械能为:22021)(21Bmv l l k +− ,由系统机械能守恒有:Am22021)(21Bmv l l k l g m +−=,所以 20)(2l l m k gl v B −−=6.如图所示,质量m = 2 kg 的物体从静止开始,沿1 / 4圆弧从A 滑到B ,在B 处速度的大小为1s m 6−⋅=v , 已知圆的半径R = 4m ,则物体从A 到B 的过程中摩擦力对它所作的功A = )J (4.42−。
有人同学是-44J 也可以算对。
解:以B 点为重力势能零点,由功能原理,摩擦力作的功为)J (4.4248.9262212122−=××−××=−=mgR mv A四、计算题:1.一个力作用在一个kg 3.0的类质点物体上,物体的位置作为时间的函数为32 1.04.0-3.0t t t x +=,式中x 以m 为单位,t 以s 为单位。
求:在t =0 s 至t =4.0 s 的时间间隔内,该力对物体做的功?解:该题两种解法:其一:用变力做功233.0 4.0 1.0x t t t =−+得出:()dt t t dx 2383+−=2383t t dt dx v +−== t dtdv a 68+−==()dt t t t dx ma dx F dA 7224621654..23−+−===()()J 5287224621654423=−+−==∫∫dt t t t dA A其二:用动能定理2.某弹簧不遵守胡克定律,若施力F ,则相应伸长为x ,力与伸长量的关系为)SI (30502x x F +=,求: (1)将弹簧从定长m 50.01=x 拉伸到定长m 00.12=x 时,外力所需做的功。
(2)将弹簧放在水平光滑的桌面上,一端固定,另一端系一个质量为6.0kg 的物体,然后将弹簧拉伸到一定长m 00.12=x ,再将物体由静止释放,求当弹簧回到m 50.01=x 时物体的速率。
(3)此弹簧的弹力是保守力吗?解:(1) 由功定义,弹簧从定长m 50.01=x 拉伸到定长m 00.12=x 时,弹力F 的作功为)J (27d )3050(d d 00.150.0221−=∫+−∫=⋅−=∫⋅=xx x x F r F A x x v v弹力的大小等于拉伸外力的大小,方向相反,故外力所需做的功为)J (27。
(2) 外力f 将弹簧拉伸到一定长m 00.12=x 时,由功能原理有弹簧和物体组成的系统机械能增量为:)J (27d )3050(d d 00.150.0221=∫+=∫⋅=∫⋅=Δx x x x f r f E xx v v 故当弹簧又回到m 50.01=x 时,此机械能增量转换为物体运动动能,有系统机械能守恒,即 221mv E =Δ 于是此时物体的速率为)s (m 30.627221−⋅=×=Δ=m Ev(3) 因为弹力F 满足∫=⋅0d r F vv ,所以此弹簧的弹力是保守力。