高中物理重難點知識突破（含詳細的例題及解析）

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 獨坐孤舟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? 是不是瘋了？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 不幸中之萬幸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? 英國人講的是英語嗎？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 最佳美術(shù)評論家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 禁止停車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! 出租汽車！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? 足球還是水球？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 購物變得很方便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 沖出黑暗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高中物理重難點知識突破（主要包括：高中物理的力、功與能、電學實驗

35、、帶電粒子在磁場中的運動部分，有詳細的例題解析和總結(jié)）一力一、難點形成原因：1、力是物體間的相互作用。受力分析時，這種相互作用只能憑著各力的產(chǎn)生條件和方向要求，再加上抽象的思維想象去畫，不想實物那么明顯，這對于剛升入高中的學生來說，多習慣于直觀形象，缺乏抽象的邏輯思惟，所以形成了難點。2、有些力的方向比較好判斷，如：重力、電場力、磁場力等，但有些力的方向難以確定。如：彈力、摩擦力等，雖然發(fā)生在接觸處，但在接觸的地方是否存在、方向如何卻難以把握。3、受力分析時除了將各力的產(chǎn)生要求、方向的判斷方法熟練掌握外，同時還要與物體的運動狀態(tài)相聯(lián)系，這就需要一定的綜合能力。由于學生對物理知識掌握不全，導致綜

36、合分析能力下降，影響了受力分析準確性和全面性。4、教師的教學要求和教學方法不當造成難點。教學要求不符合學生的實際，要求過高，想一步到位，例如：一開始就給學生講一些受力個數(shù)多、且又難以分析的物體的受力情況等。這樣勢必在學生心理上會形成障礙。二、難點突破策略：物體的受力情況決定了物體的運動狀態(tài)，正確分析物體的受力，是研究力學問題的關(guān)鍵。受力分析就是分析物體受到周圍其它物體的作用。為了保證分析結(jié)果正確，應從以下幾個方面突破難點。1.受力分析的方法：整體法和隔離法整體法隔離法概念將幾個物體作為一個整體來分析的方法將研究對象與周圍物體分隔開的方法選用原則研究系統(tǒng)外的物體對系統(tǒng)整體的作用力研究系統(tǒng)內(nèi)物體之

37、間的相互作用力注意問題分析整體周圍其他物體對整體的作用。而不畫整體內(nèi)部物體間的相互作用。分析它受到周圍其他物體對它的作用力2.受力分析的依據(jù)：各種性質(zhì)力的產(chǎn)生條件及各力方向的特點3.受力分析的步驟 ：為了在受力分析時不多分析力，也不漏力，一般情況下按下面的步驟進行： （1）確定研究對象 可以是某個物體也可以是整體。（2）按順序畫力a先畫重力：作用點畫在物體的重心，方向豎直向下。b次畫已知力c再畫接觸力（彈力和摩擦力）：看研究對象跟周圍其他物體有幾個接觸點（面），先對某個接觸點（面）分析，若有擠壓，則畫出彈力，若還有相對運動或相對運動的趨勢，則再畫出摩擦力。分析完一個接觸點（面）后，再依次分析其

38、他的接觸點（面）。d再畫其他場力：看是否有電、磁場力作用，如有則畫出。（3）驗證： a每一個力都應找到對應的施力物體 b.受的力應與物體的運動狀態(tài)對應。說明：（1）只分析研究對象受的根據(jù)性質(zhì)命名的實際力(如：重力、彈力、摩擦力等)，不畫它對別的物體的作用力。（2）合力和分力不能同時作為物體所受的力。（3）每一個力都應找到施力物體，防止“漏力”和“添力”。（4）可看成質(zhì)點的物體，力的作用點可畫在重心上，對有轉(zhuǎn)動效果的物體，則力應畫在實際位置上。（5）為了使問題簡化，常忽略某些次要的力。如物體速度不大時的空氣阻力、物體在空氣中所受的浮力等。 （6）分析物體受力時，除了考慮它與周圍物體的作用外，還要

39、考慮物體的運動情況(平衡狀態(tài)、加速或減速)，當物體的運動情況不同時，其情況也不同。4. 受力分析的輔助手段（1）物體的平衡條件（共點力作用下物體的平衡條件是合力為零）（2）牛頓第二定律（物體有加速度時）（3）牛頓第三定律（內(nèi)容：兩個物體之間的作用力和反作用力總是大小相等，方向相反，作用在一條直線上）5.常見的錯誤及防范的辦法：（1）多畫力。a.研究對象不明，錯將其他物體受到的力畫入。b.虛構(gòu)力，將不存在的力畫入。c.將合力和分力重復畫入。要防止多畫力。第一，徹底隔離研究對象。第二，每畫一個力要心中默念受力物體和施力物體。 (2) 少畫力。 少畫力往往是由受力分析過程混亂所致，因此a.要嚴格按順

40、序分析。b.分析彈力和摩擦力時，所有接觸點都要分析到。(3) 錯畫力。即把力的方向畫錯。防范辦法是要按規(guī)律作 三、分類例析1彈力有、無的判斷彈力的產(chǎn)生條件是接觸且發(fā)生彈性形變。但有的形變明顯，有的不明顯。那么如何判斷相互接觸的物體間有無彈力？法1： “假設法”，即假設接觸物體撤去，判斷研究對象是否能維持現(xiàn)狀。若維持現(xiàn)狀則接觸物體對研究對象沒有彈力，因為接觸物體使研究對象維持現(xiàn)狀等同于沒有接觸物，即接觸物形同虛設，故沒有彈力。若不能維持現(xiàn)狀則有彈力，因為接觸物撤去隨之撤去了應該有的彈力，從而改變了研究對象的現(xiàn)狀?？梢娊佑|物對研究對象維持現(xiàn)狀起著舉足輕重的作用，故有彈力。圖11ab例1：如圖所示，

41、判斷接觸面對球有無彈力，已知球靜止，接觸面光滑。【審題】在a、b圖中，若撤去細線，則球都將下滑，故細線中均有拉力, a圖中若撤去接觸面，球仍能保持原來位置不動，所以接觸面對球沒有彈力；b圖中若撤去斜面，球就不會停在原位置靜止，所以斜面對小球有支持力。 【解析】圖a中接觸面對球沒有彈力；圖b中斜面對小球有支持力法2：根據(jù)“物體的運動狀態(tài)”分析彈力。即可以先假設有彈力，分析是否符合物體所處的運動狀態(tài)?；蛘哂晌矬w所處的運動狀態(tài)反推彈力是否存在?？傊?，物體的受力必須與物體的運動狀態(tài)符合。同時依據(jù)物體的運動狀態(tài)，由二力平衡（或牛頓第二定律）還可以列方程求解彈力。圖12例2：如圖所示，判斷接觸面mo、on

42、對球有無彈力，已知球靜止，接觸面光滑。圖13【審題】圖中球由于受重力，對水平面on一定有擠壓，故水平面on對球一定有支持力，假設還受到斜面mo的彈力，如圖13所示，則球?qū)⒉粫o止，所以斜面mo對球沒有彈力?！窘馕觥克矫鎜n對球有支持力，斜面mo對球沒有彈力。再如例1的a圖中，若斜面對球有彈力，其方向應是垂直斜面且指向球，這樣球也不會處于靜止狀態(tài)，所以斜面對球也沒有彈力作用。【總結(jié)】彈力有、無的判斷是難點，分析時常用“假設法”并結(jié)合“物體的運動狀態(tài)”分析。2.彈力的方向彈力是發(fā)生彈性形變的物體由于要恢復原狀，而對它接觸的物體產(chǎn)生的力的作用 。所以彈力的方向為物體恢復形變的方向。平面與平面、點、

43、曲面接觸時，彈力方向垂直于平面，指向被壓或被支持的物體；曲面與點、曲面接觸時，彈力方向垂直于過接觸點的曲面的切面，特殊的曲面，如圓面時，彈力方向指向圓心。彈力方向與重心位置無關(guān)。 繩子的彈力方向為：沿著繩子且指向繩子收縮的方向；且同一條繩子內(nèi)各處的彈力相等 桿產(chǎn)生的彈力方向比較復雜，可以沿桿指向桿伸長或收縮的方向，也可不沿桿，與桿成一定的夾角。例3：如圖14所示，畫出物體a 所受的彈力圖14abca圖中物體a靜止在斜面上b圖中桿a靜止在光滑的半圓形的碗中c圖中a球光滑 o為圓心， o為重心?！緦忣}】圖a中接觸處為面面接觸，由于物體受重力作用，會對斜面斜向下擠壓，斜面要恢復形變，應垂直斜面斜向上

44、凸起，對物體有垂直斜面且指向物體斜向上的彈力。圖b中b處為點與曲面接觸，發(fā)生的形變?yōu)檠匕霃椒较蛳蛲獍?，要恢復形變就得沿半徑向上凸起，c處為點與平面接觸， c處碗的形變的方向為斜向下壓，要恢復形變就得沿垂直桿的方向向上，所以b處桿受的彈力為垂直過接觸點的切面沿半徑指向圓心，c處桿受的彈力為垂直桿向上。圖15abc圖c中接觸處為點與曲面接觸，發(fā)生的形變均為沿半徑分別向下凹，要恢復形變就得沿半徑方向向上凸起，所以在m、n兩接觸處對a球的彈力為垂直過接觸點的切面沿半徑方向向上，作用線均過圓心o，而不過球的重心o?！窘馕觥咳鐖D15所示【總結(jié)】彈力的方向為物體恢復形變的方向。分析時首先應明確接觸處發(fā)生的形

45、變是怎樣的，恢復形變時應向哪個方向恢復。另外應記住平面與平面、點、曲面接觸，曲面與點、曲面接觸，繩、桿彈力方向的特點，才能得以正確分析。例4：如圖16所示，小車上固定著一根彎成角的曲桿，桿的另一端固定一個質(zhì)量為m的球，試分析下列情況下桿對球的彈力的大小和方向：（1）小車靜止；（2）小車以加速度a水平向右運動；（3）小車以加速度a水平向左運動。圖16圖17【審題】此題桿對球的彈力與球所處的運動狀態(tài)有關(guān)。分析時應根據(jù)不同的運動狀態(tài)具體分析。（1）小車靜止時，球處于平衡狀態(tài)，所受合外力為零，因重力豎直向下，所以桿對球的彈力f豎直向上，大小等于球的重力mg，如圖17甲所示。（2）當小車向右加速運動時，

46、因球只受彈力和重力，所以由牛頓第二定律f=ma得，兩力的合力一定是水平向右。由平行四邊形法則得，桿對球的彈力f的方向應斜向右上方，設彈力f與豎直方向的夾角為，則由三角知識得：f= tan=a/g 如圖17乙所示。 （3）當小車向左加速運動時，因球只受彈力和重力，所以由牛頓第二定律f=ma得，兩力的合力一定是水平向左，由平行四邊形法則得，桿對球的彈力f的方向應斜向左上方，設彈力f與豎直方向的夾角為，則由三角知識得：f= tan=a/g 如圖17丙所示可見，彈力的方向與小車運動的加速度的大小有關(guān)，并不一定沿桿的方向。【解析】（1）球處于平衡狀態(tài)，桿對球產(chǎn)生的彈力方向豎直向上，且大小等于球的重力mg

47、。（2）當小車向右加速運動時，球受合力方向一定是水平向右，桿對球的彈力方向應斜向右上方，與小車運動的加速度的大小有關(guān)，其方向與豎直桿成arctan a/g角，大小等于。（3）當小車向左加速運動時，球受合力方向一定是水平向左，桿對球的彈力方向應斜向左上方，與小車運動的加速度的大小有關(guān)，其方向與豎直桿成arctan a/g角，大小等于。【總結(jié)】桿對球的彈力方向不一定沿桿，只有當加速度向右且a= gtan時，桿對小球的彈力才沿桿的方向，所以在分析物體與桿固定連接或用軸連接時，物體受桿的彈力方向應與運動狀態(tài)對應并根據(jù)物體平衡條件或牛頓第二定律求解。3判斷摩擦力的有、無摩擦力的產(chǎn)生條件為：（1）兩物體相

48、互接觸，且接觸面粗糙；（2）接觸面間有擠壓；（3）有相對運動或相對運動趨勢例5：如圖18所示，判斷下列幾種情況下物體a與接觸面間有、無摩擦力。圖a中物體a靜止 圖b中物體a沿豎直面下滑，接觸面粗糙圖c中物體a沿光滑斜面下滑圖18圖d中物體a靜止【審題】圖a中物體a靜止，水平方向上無拉力，所以物體a與接觸面間無相對運動趨勢，所以無摩擦力產(chǎn)生；圖b中物體a沿豎直面下滑時，對接觸面無壓力，所以不論接觸面是否光滑都無摩擦力產(chǎn)生；圖c中接觸面間光滑，所以無摩擦力產(chǎn)生；圖d中物體a靜止，由于重力作用，有相對斜面向下運動的趨勢，所以有靜摩擦力產(chǎn)生?！窘馕觥繄Da、圖b、圖c中無摩擦力產(chǎn)生，圖d有靜摩擦力產(chǎn)生。

49、【總結(jié)】判斷摩擦力的有、無，應依據(jù)摩擦力的產(chǎn)生條件，關(guān)鍵是看有沒有相對運動或相對運動趨勢。4摩擦力的方向摩擦力的方向為與接觸面相切，.與物體間的相對運動方向或相對運動趨勢的方向相反。但相對運動趨勢不如相對運動直觀，具有很強的隱蔽性，常用下列方法判斷。法1：“假設法”。即假設接觸面光滑，看原來相對靜止的物體間能發(fā)生怎樣的相對運動。若能發(fā)生，則這個相對運動的方向就為原來靜止時兩物體間的相對運動趨勢的方向。若不能發(fā)生，則物體間無相對運動趨勢。例6：如圖19所示為皮帶傳送裝置，甲為主動輪，傳動過程中皮帶不打滑，p、q分別為兩輪邊緣上的兩點，下列說法正確的是：ap、q兩點的摩擦力方向均與輪轉(zhuǎn)動方向相反b

50、p點的摩擦力方向與甲輪的轉(zhuǎn)動方向相反，q點的摩擦力方向與乙輪的轉(zhuǎn)動方向相同cp點的摩擦力方向與甲輪的轉(zhuǎn)動方向相同，q點的摩擦力方向與乙輪的轉(zhuǎn)動方向相反dp、q兩點的摩擦力方向均與輪轉(zhuǎn)動方向相同圖19【審題】本題可用“假設法”分析。由題意可知甲輪與皮帶間、乙輪與皮帶間均相對靜止，皮帶與輪間的摩擦力為靜摩擦力。假設甲輪是光滑的，則甲輪轉(zhuǎn)動時皮帶不動，輪上p點相對于皮帶向前運動，可知輪上p點相對于皮帶有向前運動的趨勢，則輪子上的p點受到的靜摩擦力方向向后，即與甲輪的轉(zhuǎn)動方向相反，再假設乙輪是光滑的，則當皮帶轉(zhuǎn)動時，乙輪將會靜止不動，這時，乙輪邊緣上的q點相對于皮帶向后運動，可知輪上q點有相對于皮帶向

51、后運動的趨勢，故乙輪上q點所受摩擦力向前，即與乙輪轉(zhuǎn)動方向相同?！窘馕觥空_答案為b【總結(jié)】判斷摩擦力的有、無及摩擦力的方向可采用“假設法”分析。摩擦力方向與物體間的相對運動方向或相對運動趨勢的方向相反，但不一定與物體的運動方向相反，有時還與物體的運動方向相同。圖110例7：如圖110所示，物體a疊放在物體b上，水平地面光滑，外力f作用于物體b上使它們一起運動，試分析兩物體受到的靜摩擦力的方向?！緦忣}】本題中假設a、b間接觸面是光滑的，當f使物體b向右加速時，物體a由于慣性將保持原來的靜止狀態(tài)，經(jīng)很短時間后它們的相對位置將發(fā)生變化，即物體a相對b有向左的運動，也就是說在原來相對靜止時，物體a相

52、對于b有向左的運動趨勢，所以a受到b對它的靜摩擦力方向向右（與a的實際運動方向相同）。同理b相對a有向右運動的趨勢，所以b受到a對它的靜摩擦力方向向左（與b的實際運動方向相反）。【解析】物體a相對于b有向左的運動趨勢，所以a受到b對它的靜摩擦力方向向右（與a的實際運動方向相同）。物體b相對a有向右運動的趨勢，所以b受到a對它的靜摩擦力方向向左（與b的實際運動方向相反）。如圖111所示圖111法2：根據(jù)“物體的運動狀態(tài)”來判定。即先判明物體的運動狀態(tài)（即加速度的方向），再利用牛頓第二定律（f=ma）確定合力，然后通過受力分析確定靜摩擦力的大小和方向。圖112例8：如圖112所示，a、b兩物體豎直

53、疊放在水平面上，今用水平力f拉物體，兩物體一起勻速運動，試分析a、b間的摩擦力及b與水平面間的摩擦力?！緦忣}】本題分析摩擦力時應根據(jù)物體所處的運動狀態(tài)。以a物體為研究對象：a物體在豎直方向上受重力和支持力，二者平衡，假設在水平方向上a受到b對它的靜摩擦力，該力的方向一定沿水平方向，這樣無論靜摩擦力方向向左或向右，都不可能使a物體處于平衡狀態(tài)，這與題中所給a物體處于勻速運動狀態(tài)相矛盾，故a物體不受b對它的靜摩擦力。反過來，b物體也不受a物體對它的靜摩擦力。分析b物體與水平面間的摩擦力可以a、b整體為研究對象。因a、b一起勻速運動，水平方向上合外力為零。水平方向上整體受到向右的拉力f作用，所以水平

54、面對整體一定有向左的滑動摩擦力，而水平面對整體的滑動摩擦力也就是水平面對b物體的滑動摩擦力?！窘馕觥糠治鲆娚?，因a勻速運動，所以a、b間無靜摩擦力，又因a、b整體勻速運動，由平衡條件得，物體b受到水平面對它的滑動摩擦力應向左。法3：利用牛頓第三定律來判定此法關(guān)鍵是抓住“力是成對出現(xiàn)的”，先確定受力較少的物體受到的靜摩擦力的方向，再確定另一物體受到的靜摩擦力的方向。例6中地面光滑，f使物體a、b一起向右加速運動，a物體的加速度和整體相同，由牛頓第二定律f=ma得a物體所受合外力方向一定向右，而a物體在豎直方向上受力平衡，所以水平方向上受的力為它的合外力，而在水平方向上只有可能受到b對它的靜摩擦力

55、，所以a受到b對它的靜摩擦力方向向右。b對a的摩擦力與a對b的摩擦力是一對作用力和反作用力，根據(jù)牛頓第三定律，b受到a對它的靜摩擦力方向向左?！究偨Y(jié)】靜摩擦力的方向與物體間相對運動趨勢方向相反，判斷時除了用“假設法”外，還可以根據(jù)“物體的運動狀態(tài)”、及 牛頓第三定律來分析?；瑒幽Σ亮Φ姆较蚺c物體間相對運動的方向相反。5物體的受力分析例9：如圖113甲所示，豎直墻壁光滑，分析靜止的木桿受哪幾個力作用?！緦忣}】首先選取研究對象木桿，其次按順序畫力：重力作用在木桿的中點，方向豎直向下；畫彈力。有兩個接觸點，墻與桿接觸點屬點面接觸，彈力垂直于墻且指向桿，地與桿的接觸點也屬點面接觸，桿受的彈力垂直于地面

56、且指向桿；畫摩擦力。豎直墻光滑，墻與桿接觸點沒有摩擦力；假設地面光滑，桿將會向右運動，所以桿靜止時有相對地面向右的運動趨勢，所以地面對桿有向左的摩擦力?！窘馕觥織U受重力、方向豎直向下；彈力n1，垂直于墻且指向桿，彈力n2，垂直于地面且指向桿；地面對桿向左的摩擦力f。如圖113乙所示乙圖113【總結(jié)】受力分析時應按步驟分析，桿受的各力應畫在實際位置上。不要將各力的作用點都移到重心上去。例10：如圖114甲所示，、疊放于水平地面上，加一水平力，三物體仍靜止，分析、的受力情況。【審題】用隔離法分析：先取為研究對象：受向下的重力、對的支持力。假設b對a有水平方向的摩擦力，不論方向水平向左還是向右，都與

57、a處的靜止狀態(tài)相矛盾，所以對沒有摩擦力。取為研究對象：受向下的重力、對的壓力、對的支持力、水平力。因處靜止，水平方向受合力為零，根據(jù)平衡條件，對一定有水平向左的摩擦力f。再取為研究對象：受向下的重力、對的壓力，地面對的支持力，由牛頓第三定律得，對的摩擦力向右，因處靜止合力為零，根據(jù)平衡條件，地對的摩擦力f一定水平向左?！窘馕觥縜、b、c三物體的受力如圖圖114乙所示甲乙圖114【總結(jié)】用隔離法分析物體受力分析最常用的方法，分析時應將研究的物體單獨拿出來，不要都畫在一起，以免出現(xiàn)混亂。同時應根據(jù)牛頓第三定律分析。a對b的壓力及b對c的壓力應以和表示，不要用和表示，因中它們跟、是不同的。此題也可以用先整