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1、exclaimed.just then, a man hurried to the ticket-office.can i return these two tickets? he asked.certainly, the girl said.could i have those two tickets please ? i asked.certainly, the girl said, but they are for next wednesdays performance.i might as well have them, i said sadly.lesson20 one man in
2、 a boat 独坐孤舟fishing is my favourite sport. i often fish for hours without catching anything. but this does not worry me. some fishermen are unlucky. instead of catching fish, they catch old boots and rubbish. i am even less lucky. i never catch anything-not even old boots. after having spent whole m
3、ornings on the river, i always go home with an empty bag. you must give up fishing! my friends say. its a waste of time. but they dont realize one important thing. im not really interested in fishing. i am only interested in sitting in a boat and doing nothing at all !lesson21 mad or not? 是不是疯了?aero
4、planes are slowly driving me mad. i live near an airport and passing planes can be heard night and day. the airport was built during the war, but for some reason it could not be used then. last year, however, it came into use. over a hundred people must have been driven away from their homes by the
5、noise. i am one of the few people left. sometimes i think this house will be knocked down by a passing plane. i have been offered a large sum of money to go away, but i am determined to stay here. everybody says i must be mad and they are probably right.lesson22 a glass envelope 玻璃信封my daughter, jan
6、e, never dreamed of receiving a letter from a girl of her own age in holland. last year, we were travelling across the channel and jane put a piece of paper with her name and address on it into a bottle. she threw the bottle into the sea. she never thought of it again, but ten months later, she rece
7、ived a letter from a girl in holland. both girls write to each other regularly now. however, they have decided to use the post-office. letters will cost a litt1e more, but they will certainly travel faster. lesson23 a new house 新居i had a letter from my sister yesterday. she lives in nigeria. in her
8、letter, she said that she would come to england next year. if she comes, she will get a surprise. we are now living in a beautiful new house in the country. work on it had begun before my sister left. the house was completed five months ago. in my letter, i told her that she could stay with us. the
9、house has many large rooms and there is a lovely garden. it is a very modern house, so it looks strange to some people. it must be the only modern house in the district.lesson24 it could be worse 不幸中之万幸i entered the hotel managers office and sat down. i had just lost &50 and i felt very upset. i lef
10、t the money in my room, i said, and its not there now. the manager was sympathetic, but he could do nothing. everyones losing money these days, he said. he started to complain about this wicked world but was interrupted by a knock at the door. a girl came in and put an envelope on his desk. it conta
11、ined &5o. i found this outside this gentlemans room she said. well, i said to the manager, there is still some honesty in this world !lesson25 do the english speak english? 英国人讲的是英语吗?i arrived in london at last. the railway station was big, black and dark. i did not know the way to my hotel, so i as
12、ked a porter. i not only spoke english very carefully, but very clearly as well. the porter, however, could not understand me; i repeated my question several times and at last he understood. he answered me, but he spoke neither slowly nor clearly. i am a foreigner, i said. then he spoke slowly, but
13、i could not understand him. my teacher never spoke english like that! the porter and i looked at each other and smiled. then he said something and i understood it. youll soon learn english! he said. i wonder. in england, each man speaks a different language. the english understand each other, but i
14、dont understand them ! do they speak english ?lesson26 the best art critics 最佳美术评论家i am an art student and i paint a lot of pictures. many people pretend that they understand modern art. they always tell you what a picture is about. of course, many pictures arc not about anything. they are just pret
15、ty patterns. we like them in the same way that we like pretty curtain material. i think that young children often appreciate modern pictures better than anyone else. they notice more. my sister is only seven, but she always tells me whether my pictures are good or not. she came into my room yesterda
16、y. what are you doing ? she asked. im hanging this picture on the wall, i answered. its a new one. do you like it ? she looked at it critically for a moment. its all right, she said, but isnt it upside-down ? i looked at it again. she was right! it was!lesson27 a wet night 雨夜late in the afternoon, t
17、he boys put up their tent in the middle of a field. as soon as this was done, they cooked a meal over an open fire. they were all hungry and the food smelt good. after a wonderful meal, they told stories and sang songs by the camp fire. but some time later it began to rain. the boys felt tired so th
18、ey pet out the fire and crept into their tent. their sleeping-bags were warm and comfortable, so they all slept soundly. in the middle of the night, two boys woke up and began shouting. the tent was full of water! they all leapt out of their sleeping-bags and hurried outside. it was raining heavily
19、and they found that a stream had formed in the field. the stream wound its way across the field and then flowed right under their tent!lesson28 no parking 禁止停车jasper white is one of those rare people who believes in ancient myths. he has just bought a new house in the city, but ever since he moved i
20、n, he has had trouble with motorists. when he returns home at night, he always finds that someone has parked a car outside his gate. because of this, he has not been able to get his own car into his garage even once. jasper has put up no parking signs outside his gate, but these have not had any eff
21、ect. now he has put an ugly stone head over the gate. it is one of the ugliest faces i have ever seen. i asked him what it was and he told me that it was medusa, the gorgon. jasper hopes that she will turn motorists to stone. but none of them has been turned to stone yet!lesson29 taxi! 出租汽车!captain
22、ben fawcett has bought an unusual taxi and has begun a new service. the taxi is a small swiss aeroplane called a pilatus porter. this wonderful plane can carry seven passengers. the most surprising thing about it, however, is that it can land anywhere: on snow, water, or even on a ploughed field. ca
23、ptain fawcetts first passenger was a doctor who flew from birmingham to a lonely village in the welsh mountains. since then, captain fawcett has flown passengers to many unusual places. once he landed on the roof of a block of flats and on another occasion, he landed in a deserted car park. captain
24、fawcett has just refused a strange request from a businessman. the man wanted to fly to rockall, a lonely island in the atlantic ocean, but captain fawcett did not take him because the trip was too dangerous.lesson30 football or polo? 足球还是水球?the wayle is a small river that cuts across the park near
25、my home. i like sitting by the wayle on fine afternoons. it was warm last sunday, so i went and sat on the river bank as usual. some children were playing games on the bank and there were some people rowing on the river. suddenly, one of the children kicked a ball very hard and it went towards a pas
26、sing boat. some people on the bank called out to the man in the boat, but he did not hear them. the ball struck him so hard that he nearly fell into the water. i turned to look at the children, but there werent any in sight: they had all run away! the man laughed when he realized what had happened.
27、he called out to the children and threw the ball back to the bank. lesson31 success story 成功者的故事yesterday afternoon frank hawkins was telling me about his experiences as a young man. frank is now the head of a very large business company, but as a boy he used to work in a small shop. it was his job
28、to repair bicycles and at that time he used to work fourteen hours a day. he saved money for years and in 1938 he bought a small work-shop of his own. during the war frank used to make spare parts for aeroplanes. at that time he had two helpers. by the end of the war, the small work-shop had become
29、a large factory which employed seven hundred and twenty-eight people. frank smiled when he remembered his hard early years and the long road to success. he was still smiling when the door opened and his wife came in. she wanted him to repair their sons bicycle! lesson32 shopping made easy 购物变得很方便peo
30、ple are not so honest as they once were. the temptation to steal is greater than ever before-especially in large shops. a detective recently watched a well-dressed woman who always went into a large store on monday mornings. one monday, there were fewer people in the shop than usual when the woman c
31、ame in, so it was easier for the detective to watch her. the woman first bought a few small articles. after a little time, she chose one of the most expensive dresses in the shop and handed it to an assistant who wrapped it up for her as quickly as possible. then the woman simply took the parcel and
32、 walked out of the shop without paying. when she was arrested, the detective found out that the shop-assistant was her daughter. the girl gave her mother a free dress once a week !lesson33 out of the darkness 冲出黑暗nearly a week passed before the girl was able to explain what had happened to her. one
33、afternoon she set out from the coast in a small boat and was caught in a storm. towards evening, the boat struck a rock and the girl jumped into the sea. then she swam to the shore after spending the whole night in the water. during that time she covered a distance of eight miles. early next morning
34、, she saw a light ahead. she knew she was near the shore because the light was high up on the cliffs. on arriving at the shore, the girl struggled up the cliff towards the light she had seen. that was all she remembered. when she woke up a day later, she found herself高中物理重难点知识突破(主要包括:高中物理的力、功与能、电学实验
35、、带电粒子在磁场中的运动部分,有详细的例题解析和总结)一力一、难点形成原因:1、力是物体间的相互作用。受力分析时,这种相互作用只能凭着各力的产生条件和方向要求,再加上抽象的思维想象去画,不想实物那么明显,这对于刚升入高中的学生来说,多习惯于直观形象,缺乏抽象的逻辑思惟,所以形成了难点。2、有些力的方向比较好判断,如:重力、电场力、磁场力等,但有些力的方向难以确定。如:弹力、摩擦力等,虽然发生在接触处,但在接触的地方是否存在、方向如何却难以把握。3、受力分析时除了将各力的产生要求、方向的判断方法熟练掌握外,同时还要与物体的运动状态相联系,这就需要一定的综合能力。由于学生对物理知识掌握不全,导致综
36、合分析能力下降,影响了受力分析准确性和全面性。4、教师的教学要求和教学方法不当造成难点。教学要求不符合学生的实际,要求过高,想一步到位,例如:一开始就给学生讲一些受力个数多、且又难以分析的物体的受力情况等。这样势必在学生心理上会形成障碍。二、难点突破策略:物体的受力情况决定了物体的运动状态,正确分析物体的受力,是研究力学问题的关键。受力分析就是分析物体受到周围其它物体的作用。为了保证分析结果正确,应从以下几个方面突破难点。1.受力分析的方法:整体法和隔离法整体法隔离法概念将几个物体作为一个整体来分析的方法将研究对象与周围物体分隔开的方法选用原则研究系统外的物体对系统整体的作用力研究系统内物体之
37、间的相互作用力注意问题分析整体周围其他物体对整体的作用。而不画整体内部物体间的相互作用。分析它受到周围其他物体对它的作用力2.受力分析的依据:各种性质力的产生条件及各力方向的特点3.受力分析的步骤 :为了在受力分析时不多分析力,也不漏力,一般情况下按下面的步骤进行: (1)确定研究对象 可以是某个物体也可以是整体。(2)按顺序画力a先画重力:作用点画在物体的重心,方向竖直向下。b次画已知力c再画接触力(弹力和摩擦力):看研究对象跟周围其他物体有几个接触点(面),先对某个接触点(面)分析,若有挤压,则画出弹力,若还有相对运动或相对运动的趋势,则再画出摩擦力。分析完一个接触点(面)后,再依次分析其
38、他的接触点(面)。d再画其他场力:看是否有电、磁场力作用,如有则画出。(3)验证: a每一个力都应找到对应的施力物体 b.受的力应与物体的运动状态对应。说明:(1)只分析研究对象受的根据性质命名的实际力(如:重力、弹力、摩擦力等),不画它对别的物体的作用力。(2)合力和分力不能同时作为物体所受的力。(3)每一个力都应找到施力物体,防止“漏力”和“添力”。(4)可看成质点的物体,力的作用点可画在重心上,对有转动效果的物体,则力应画在实际位置上。(5)为了使问题简化,:雎阅承┐我牧。如物体速度不大时的空气阻力、物体在空气中所受的浮力等。 (6)分析物体受力时,除了考虑它与周围物体的作用外,还要
39、考虑物体的运动情况(平衡状态、加速或减速),当物体的运动情况不同时,其情况也不同。4. 受力分析的辅助手段(1)物体的平衡条件(共点力作用下物体的平衡条件是合力为零)(2)牛顿第二定律(物体有加速度时)(3)牛顿第三定律(内容:两个物体之间的作用力和反作用力总是大小相等,方向相反,作用在一条直线上)5.常见的错误及防范的办法:(1)多画力。a.研究对象不明,错将其他物体受到的力画入。b.虚构力,将不存在的力画入。c.将合力和分力重复画入。要防止多画力。第一,彻底隔离研究对象。第二,每画一个力要心中默念受力物体和施力物体。 (2) 少画力。 少画力往往是由受力分析过程混乱所致,因此a.要严格按顺
40、序分析。b.分析弹力和摩擦力时,所有接触点都要分析到。(3) 错画力。即把力的方向画错。防范办法是要按规律作 三、分类例析1弹力有、无的判断弹力的产生条件是接触且发生弹性形变。但有的形变明显,有的不明显。那么如何判断相互接触的物体间有无弹力?法1: “假设法”,即假设接触物体撤去,判断研究对象是否能维持现状。若维持现状则接触物体对研究对象没有弹力,因为接触物体使研究对象维持现状等同于没有接触物,即接触物形同虚设,故没有弹力。若不能维持现状则有弹力,因为接触物撤去随之撤去了应该有的弹力,从而改变了研究对象的现状。可见接触物对研究对象维持现状起着举足轻重的作用,故有弹力。图11ab例1:如图所示,
41、判断接触面对球有无弹力,已知球静止,接触面光滑。【审题】在a、b图中,若撤去细线,则球都将下滑,故细线中均有拉力, a图中若撤去接触面,球仍能保持原来位置不动,所以接触面对球没有弹力;b图中若撤去斜面,球就不会停在原位置静止,所以斜面对小球有支持力。 【解析】图a中接触面对球没有弹力;图b中斜面对小球有支持力法2:根据“物体的运动状态”分析弹力。即可以先假设有弹力,分析是否符合物体所处的运动状态。或者由物体所处的运动状态反推弹力是否存在。总之,物体的受力必须与物体的运动状态符合。同时依据物体的运动状态,由二力平衡(或牛顿第二定律)还可以列方程求解弹力。图12例2:如图所示,判断接触面mo、on
42、对球有无弹力,已知球静止,接触面光滑。图13【审题】图中球由于受重力,对水平面on一定有挤压,故水平面on对球一定有支持力,假设还受到斜面mo的弹力,如图13所示,则球将不会静止,所以斜面mo对球没有弹力。【解析】水平面on对球有支持力,斜面mo对球没有弹力。再如例1的a图中,若斜面对球有弹力,其方向应是垂直斜面且指向球,这样球也不会处于静止状态,所以斜面对球也没有弹力作用。【总结】弹力有、无的判断是难点,分析时常用“假设法”并结合“物体的运动状态”分析。2.弹力的方向弹力是发生弹性形变的物体由于要恢复原状,而对它接触的物体产生的力的作用 。所以弹力的方向为物体恢复形变的方向。平面与平面、点、
43、曲面接触时,弹力方向垂直于平面,指向被压或被支持的物体;曲面与点、曲面接触时,弹力方向垂直于过接触点的曲面的切面,特殊的曲面,如圆面时,弹力方向指向圆心。弹力方向与重心位置无关。 绳子的弹力方向为:沿着绳子且指向绳子收缩的方向;且同一条绳子内各处的弹力相等 杆产生的弹力方向比较复杂,可以沿杆指向杆伸长或收缩的方向,也可不沿杆,与杆成一定的夹角。例3:如图14所示,画出物体a 所受的弹力图14abca图中物体a静止在斜面上b图中杆a静止在光滑的半圆形的碗中c图中a球光滑 o为圆心, o为重心。【审题】图a中接触处为面面接触,由于物体受重力作用,会对斜面斜向下挤压,斜面要恢复形变,应垂直斜面斜向上
44、凸起,对物体有垂直斜面且指向物体斜向上的弹力。图b中b处为点与曲面接触,发生的形变为沿半径方向向外凹,要恢复形变就得沿半径向上凸起,c处为点与平面接触, c处碗的形变的方向为斜向下压,要恢复形变就得沿垂直杆的方向向上,所以b处杆受的弹力为垂直过接触点的切面沿半径指向圆心,c处杆受的弹力为垂直杆向上。图15abc图c中接触处为点与曲面接触,发生的形变均为沿半径分别向下凹,要恢复形变就得沿半径方向向上凸起,所以在m、n两接触处对a球的弹力为垂直过接触点的切面沿半径方向向上,作用线均过圆心o,而不过球的重心o。【解析】如图15所示【总结】弹力的方向为物体恢复形变的方向。分析时首先应明确接触处发生的形
45、变是怎样的,恢复形变时应向哪个方向恢复。另外应记住平面与平面、点、曲面接触,曲面与点、曲面接触,绳、杆弹力方向的特点,才能得以正确分析。例4:如图16所示,小车上固定着一根弯成角的曲杆,杆的另一端固定一个质量为m的球,试分析下列情况下杆对球的弹力的大小和方向:(1)小车静止;(2)小车以加速度a水平向右运动;(3)小车以加速度a水平向左运动。图16图17【审题】此题杆对球的弹力与球所处的运动状态有关。分析时应根据不同的运动状态具体分析。(1)小车静止时,球处于平衡状态,所受合外力为零,因重力竖直向下,所以杆对球的弹力f竖直向上,大小等于球的重力mg,如图17甲所示。(2)当小车向右加速运动时,
46、因球只受弹力和重力,所以由牛顿第二定律f=ma得,两力的合力一定是水平向右。由平行四边形法则得,杆对球的弹力f的方向应斜向右上方,设弹力f与竖直方向的夹角为,则由三角知识得:f= tan=a/g 如图17乙所示。 (3)当小车向左加速运动时,因球只受弹力和重力,所以由牛顿第二定律f=ma得,两力的合力一定是水平向左,由平行四边形法则得,杆对球的弹力f的方向应斜向左上方,设弹力f与竖直方向的夹角为,则由三角知识得:f= tan=a/g 如图17丙所示可见,弹力的方向与小车运动的加速度的大小有关,并不一定沿杆的方向。【解析】(1)球处于平衡状态,杆对球产生的弹力方向竖直向上,且大小等于球的重力mg
47、。(2)当小车向右加速运动时,球受合力方向一定是水平向右,杆对球的弹力方向应斜向右上方,与小车运动的加速度的大小有关,其方向与竖直杆成arctan a/g角,大小等于。(3)当小车向左加速运动时,球受合力方向一定是水平向左,杆对球的弹力方向应斜向左上方,与小车运动的加速度的大小有关,其方向与竖直杆成arctan a/g角,大小等于。【总结】杆对球的弹力方向不一定沿杆,只有当加速度向右且a= gtan时,杆对小球的弹力才沿杆的方向,所以在分析物体与杆固定连接或用轴连接时,物体受杆的弹力方向应与运动状态对应并根据物体平衡条件或牛顿第二定律求解。3判断摩擦力的有、无摩擦力的产生条件为:(1)两物体相
48、互接触,且接触面粗糙;(2)接触面间有挤压;(3)有相对运动或相对运动趋势例5:如图18所示,判断下列几种情况下物体a与接触面间有、无摩擦力。图a中物体a静止 图b中物体a沿竖直面下滑,接触面粗糙图c中物体a沿光滑斜面下滑图18图d中物体a静止【审题】图a中物体a静止,水平方向上无拉力,所以物体a与接触面间无相对运动趋势,所以无摩擦力产生;图b中物体a沿竖直面下滑时,对接触面无压力,所以不论接触面是否光滑都无摩擦力产生;图c中接触面间光滑,所以无摩擦力产生;图d中物体a静止,由于重力作用,有相对斜面向下运动的趋势,所以有静摩擦力产生。【解析】图a、图b、图c中无摩擦力产生,图d有静摩擦力产生。
49、【总结】判断摩擦力的有、无,应依据摩擦力的产生条件,关键是看有没有相对运动或相对运动趋势。4摩擦力的方向摩擦力的方向为与接触面相切,.与物体间的相对运动方向或相对运动趋势的方向相反。但相对运动趋势不如相对运动直观,具有很强的隐蔽性,常用下列方法判断。法1:“假设法”。即假设接触面光滑,看原来相对静止的物体间能发生怎样的相对运动。若能发生,则这个相对运动的方向就为原来静止时两物体间的相对运动趋势的方向。若不能发生,则物体间无相对运动趋势。例6:如图19所示为皮带传送装置,甲为主动轮,传动过程中皮带不打滑,p、q分别为两轮边缘上的两点,下列说法正确的是:ap、q两点的摩擦力方向均与轮转动方向相反b
50、p点的摩擦力方向与甲轮的转动方向相反,q点的摩擦力方向与乙轮的转动方向相同cp点的摩擦力方向与甲轮的转动方向相同,q点的摩擦力方向与乙轮的转动方向相反dp、q两点的摩擦力方向均与轮转动方向相同图19【审题】本题可用“假设法”分析。由题意可知甲轮与皮带间、乙轮与皮带间均相对静止,皮带与轮间的摩擦力为静摩擦力。假设甲轮是光滑的,则甲轮转动时皮带不动,轮上p点相对于皮带向前运动,可知轮上p点相对于皮带有向前运动的趋势,则轮子上的p点受到的静摩擦力方向向后,即与甲轮的转动方向相反,再假设乙轮是光滑的,则当皮带转动时,乙轮将会静止不动,这时,乙轮边缘上的q点相对于皮带向后运动,可知轮上q点有相对于皮带向
51、后运动的趋势,故乙轮上q点所受摩擦力向前,即与乙轮转动方向相同。【解析】正确答案为b【总结】判断摩擦力的有、无及摩擦力的方向可采用“假设法”分析。摩擦力方向与物体间的相对运动方向或相对运动趋势的方向相反,但不一定与物体的运动方向相反,有时还与物体的运动方向相同。图110例7:如图110所示,物体a叠放在物体b上,水平地面光滑,外力f作用于物体b上使它们一起运动,试分析两物体受到的静摩擦力的方向。【审题】本题中假设a、b间接触面是光滑的,当f使物体b向右加速时,物体a由于惯性将保持原来的静止状态,经很短时间后它们的相对位置将发生变化,即物体a相对b有向左的运动,也就是说在原来相对静止时,物体a相
52、对于b有向左的运动趋势,所以a受到b对它的静摩擦力方向向右(与a的实际运动方向相同)。同理b相对a有向右运动的趋势,所以b受到a对它的静摩擦力方向向左(与b的实际运动方向相反)。【解析】物体a相对于b有向左的运动趋势,所以a受到b对它的静摩擦力方向向右(与a的实际运动方向相同)。物体b相对a有向右运动的趋势,所以b受到a对它的静摩擦力方向向左(与b的实际运动方向相反)。如图111所示图111法2:根据“物体的运动状态”来判定。即先判明物体的运动状态(即加速度的方向),再利用牛顿第二定律(f=ma)确定合力,然后通过受力分析确定静摩擦力的大小和方向。图112例8:如图112所示,a、b两物体竖直
53、叠放在水平面上,今用水平力f拉物体,两物体一起匀速运动,试分析a、b间的摩擦力及b与水平面间的摩擦力。【审题】本题分析摩擦力时应根据物体所处的运动状态。以a物体为研究对象:a物体在竖直方向上受重力和支持力,二者平衡,假设在水平方向上a受到b对它的静摩擦力,该力的方向一定沿水平方向,这样无论静摩擦力方向向左或向右,都不可能使a物体处于平衡状态,这与题中所给a物体处于匀速运动状态相矛盾,故a物体不受b对它的静摩擦力。反过来,b物体也不受a物体对它的静摩擦力。分析b物体与水平面间的摩擦力可以a、b整体为研究对象。因a、b一起匀速运动,水平方向上合外力为零。水平方向上整体受到向右的拉力f作用,所以水平
54、面对整体一定有向左的滑动摩擦力,而水平面对整体的滑动摩擦力也就是水平面对b物体的滑动摩擦力。【解析】分析见上,因a匀速运动,所以a、b间无静摩擦力,又因a、b整体匀速运动,由平衡条件得,物体b受到水平面对它的滑动摩擦力应向左。法3:利用牛顿第三定律来判定此法关键是抓住“力是成对出现的”,先确定受力较少的物体受到的静摩擦力的方向,再确定另一物体受到的静摩擦力的方向。例6中地面光滑,f使物体a、b一起向右加速运动,a物体的加速度和整体相同,由牛顿第二定律f=ma得a物体所受合外力方向一定向右,而a物体在竖直方向上受力平衡,所以水平方向上受的力为它的合外力,而在水平方向上只有可能受到b对它的静摩擦力
55、,所以a受到b对它的静摩擦力方向向右。b对a的摩擦力与a对b的摩擦力是一对作用力和反作用力,根据牛顿第三定律,b受到a对它的静摩擦力方向向左。【总结】静摩擦力的方向与物体间相对运动趋势方向相反,判断时除了用“假设法”外,还可以根据“物体的运动状态”、及 牛顿第三定律来分析。滑动摩擦力的方向与物体间相对运动的方向相反。5物体的受力分析例9:如图113甲所示,竖直墙壁光滑,分析静止的木杆受哪几个力作用。【审题】首先选取研究对象木杆,其次按顺序画力:重力作用在木杆的中点,方向竖直向下;画弹力。有两个接触点,墙与杆接触点属点面接触,弹力垂直于墙且指向杆,地与杆的接触点也属点面接触,杆受的弹力垂直于地面
56、且指向杆;画摩擦力。竖直墙光滑,墙与杆接触点没有摩擦力;假设地面光滑,杆将会向右运动,所以杆静止时有相对地面向右的运动趋势,所以地面对杆有向左的摩擦力。【解析】杆受重力、方向竖直向下;弹力n1,垂直于墙且指向杆,弹力n2,垂直于地面且指向杆;地面对杆向左的摩擦力f。如图113乙所示乙图113【总结】受力分析时应按步骤分析,杆受的各力应画在实际位置上。不要将各力的作用点都移到重心上去。例10:如图114甲所示,、叠放于水平地面上,加一水平力,三物体仍静止,分析、的受力情况。【审题】用隔离法分析:先取为研究对象:受向下的重力、对的支持力。假设b对a有水平方向的摩擦力,不论方向水平向左还是向右,都与
57、a处的静止状态相矛盾,所以对没有摩擦力。取为研究对象:受向下的重力、对的压力、对的支持力、水平力。因处静止,水平方向受合力为零,根据平衡条件,对一定有水平向左的摩擦力f。再取为研究对象:受向下的重力、对的压力,地面对的支持力,由牛顿第三定律得,对的摩擦力向右,因处静止合力为零,根据平衡条件,地对的摩擦力f一定水平向左。【解析】a、b、c三物体的受力如图图114乙所示甲乙图114【总结】用隔离法分析物体受力分析最常用的方法,分析时应将研究的物体单独拿出来,不要都画在一起,以免出现混乱。同时应根据牛顿第三定律分析。a对b的压力及b对c的压力应以和表示,不要用和表示,因中它们跟、是不同的。此题也可以
58、用先整体后部分,由下向上的方法分析。例11:如图115甲所示,物体、静止,画出、的受力图。 【审题】用隔离法分析。先隔离:受重力,外力,由于f的作用,b和a之间的挤压,所以对有支持力ab,假设a、b接触面光滑,物体b将相对a下滑,所以有相对a向下的运动趋势,b受向上的静摩擦力fab。再隔离:受重力,墙对的支持力墙,由牛顿第三定律得,a受到b对它的压力nba,水平向左,摩擦力fba,方向竖直向下。假设墙是光滑的,物体相对墙将下滑,也就是说物体相对墙有向下的运动趋势,所以墙对有竖直向上的摩擦力f墙。【解析】a、b受力如图115乙所示图115甲图115乙总结:此类问题用隔离法分析,应注意a、b间、a
59、与墙间的摩擦力的分析,同时要根据牛顿第三定律分析。图116例12:如图116所示,用两相同的夹板夹住三个重为g的物体a、b、c,三个物体均保持静止,请分析各个物体的受力情况.【审题】要分析各物体的受力情况,关键是分析a、b间、b、c间是否有摩擦力,所以可用先整体后隔离的方法。首先以三物体为一整体。竖直方向上,受重力3g,竖直向下,两板对它向上的摩擦力,分别为f;水平方向上,受两侧板对它的压力n1、n2。根据平衡条件得,每一侧受的摩擦力大小等于1.5g。然后再用隔离法分析a、b、c的受力情况,先隔离a,a物体受重力g,方向竖直向下,板对它的向上的摩擦力f,大小等于1.5g ,a物体要平衡,就必须
60、受到一个b对它的向下的摩擦力fba,根据平衡条件得,大小应等于0.5 g , 水平方向上,a物体受板对它的压力n1和b对它的压力nba; 再隔离c,c物体的受力情况与a物体类似. 竖直方向上受重力g、板对它的向上的摩擦力f、b对它的向下的摩擦力fbc,水平方向上受板对它的压力n2、b对它的压力nbc。再隔离b,竖直方向上b物体受重力g 、由牛顿第三定律得,b受到a对它的向上的摩擦力fab 、c对它的向上的摩擦力fcb ,以及水平方向上a对它的压力nab和c对它的压力ncb。【解析】a、b、c受力如图图117所示图117【总结】明确各物体所受的摩擦力是解决此类问题的关键,较好的解决方法是先整体法
61、确定两侧的摩擦力,再用隔离法确定单个物体所受的摩擦力。例13:如图118所示,放置在水平地面上的直角劈m上有一个质量为m的物体,若m在其上匀速下滑,仍保持静止,那么正确的说法是( )图118a.对地面的压力等于(m+m)gb.对地面的压力大于(m+m)g c.地面对没有摩擦力d.地面对有向左的摩擦力【审题】先用隔离法分析。先隔离m,m受重力mg、斜面对它的支持力n、沿斜面向上的摩擦力f,因m沿斜面匀速下滑,所以支持力n和沿斜面向上的摩擦力f可根据平衡条件求出。再隔离m,m受竖直向下重力mg、地面对它竖直向上的支持力n地、由牛顿第三定律得,m对m有垂直斜面向下的压力n和沿斜面向下的摩擦力f,m相
62、对地面有没有运动趋势,关键看f和n在水平方向的分量是否相等,若二者相等,则m相对地面无运动趋势,若二者不相等,则m相对地面有运动趋势,而摩擦力方向应根据具体的相对运动趋势的方向确定。图119甲乙【解析】m、m的受力如图119所示对m:建系如图甲所示,因m沿斜面匀速下滑,由平衡条件得:支持力n=mgcos,摩擦力f=mgsin对m:建系如图乙所示,由牛顿第三定律得,n= n,f= f,在水平方向上,压力n的水平分量nsin= mgcossin,摩擦力f的水平分量fcos= mgsincos,可见fcos=nsin,所以m相对地面没有运动趋势,所以地面对m没有摩擦力。 在竖直方向上,整体平衡,由平
63、衡条件得:n地= fsin+ ncos+mg=mg+mg。所以正确答案为:a、c再以整体法分析:对地面的压力和地面对m的支持力是一对作用力和反作用力,大小相等,方向相反。而地面对m的支持力、地面对摩擦力是m和m整体的外力,所以要讨论这两个问题,可以整体为研究对象。整体在竖直方向上受到重力和支持力,因m在斜面上匀速下滑、m静止不动,即整体处于平衡状态,所以竖直方向上地面对m的支持力等于重力,水平方向上若受地面对m的摩擦力,无论摩擦力的方向向左还是向右,水平方向上整体都不能处于平衡,所以整体在水平方向上不受摩擦力。图120【解析】整体受力如图120所示,正确答案为:a、c。【总结】综上可见,在分析整体受的外力时,以整体为研究对象分析比较简单。也可以隔离法分析,但较麻烦,在实际解题时,可灵活应用整体法和隔离法,将二者有机地结合起来。总之,在进行受力分析时一定要按次序画出物体实际受的各个力,为解决这一难
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