最新人教版八年級下冊物理《力》完整講義

1、專業(yè)好文檔第七章 力第1節(jié) 力1、 生活中存在的力 拉弓射箭 運(yùn)動員舉起杠鈴 運(yùn)動員投籃 壓路機(jī)壓路思考：什么是力？人 弓推土機(jī) 土運(yùn)動員 杠鈴物 體 物體2、力的概念：力是物體對物體的作用 受力物體： 施力物體： 3、力產(chǎn)生的條件：必須有兩個或兩個以上的物體。物體間必須有相互作用（可以不接觸）。4、力的性質(zhì)：物體間力的作用是相互的（相互作用力在任何情況下都是大小相等，方向相反，作用在不同物體上）。兩物體相互作用時，施力物體同時也是受力物體，反之，受力物體同時也是施力物體。思考：力的作用效果有哪些？ 用力扳竹子 小孩盤腿坐在木板上 駱駝在沙地上踩出腳印 足球運(yùn)動中相互傳球 守門員守球5、力的作

2、用效果：力可以改變物體的運(yùn)動狀態(tài) 力可以改變物體的形狀說明：物體的運(yùn)動狀態(tài)是否改變一般指：物體的運(yùn)動快慢是否改變（速度大小的改變）和物體的運(yùn)動方向是否改變例1：在跳板跳水這個運(yùn)動項目中運(yùn)動員對跳板施力的同時，也受到跳板對他的作用力但這兩個力的作用效果是不一樣的。前者要改變了跳板的 ，后者主要改變了運(yùn)動員的 例2：下列實例中，力的作用下使物體的形狀發(fā)生變化的是：（ ）a：緊急剎車。b：騎自行車加速前進(jìn)。c：做直線運(yùn)動的足球碰到球員后，運(yùn)動方向發(fā)生改變。d：兩手用力扳竹條，使其彎曲。點撥：理解力的概念應(yīng)把握兩條：1、力就是作用，力現(xiàn)象中的物體間推、拉、提、壓、排斥、吸引等都叫力的作用。2、力不能離

3、開物體而存在，有力至少有兩個物體，一個是施力物體，另外一個是受力物體。運(yùn)動狀態(tài)的理解：1、由于物體的運(yùn)動和靜止是相對的，因此我們把靜止的當(dāng)作是一個特殊的運(yùn)動狀態(tài)。2、一個物體保持靜止或做勻速直線運(yùn)動，我們就說它的運(yùn)動狀態(tài)不變，否則我們就說它的運(yùn)動狀態(tài)變了。6、力的單位：國際單位制中力的單位是牛頓簡稱牛，用n 表示。力的感性認(rèn)識：拿兩個雞蛋所用的力大約1n。7、力的測量：測力計：測量力的大小的工具。分類：彈簧測力計、握力計。彈簧測力計：a、原理：在彈性限度內(nèi)，彈簧的伸長與所受的拉力成正比。b、使用方法：(1)要檢查指針是否指在零刻度，如果不是，則要調(diào)零；(2)認(rèn)清最小刻度和測量范圍；(3)輕拉秤

4、鉤幾次，看每次松手后，指針是否回到零刻度，(4)測量時彈簧測力計內(nèi)彈簧的軸線與所測力的方向一致；(5)觀察讀數(shù)時，視線必須與刻度盤垂直。(6)測量力時不能超過彈簧測力計的量程。c、注意事項：加在彈簧測力計上的力不許超過它的最大量程。d、物理實驗中,有些物理量的大小是不宜直接觀察的,但它變化時引起其他物理量的變化卻容易觀察,用容易觀察的量顯示不宜觀察的量,是制作測量儀器的一種思路。這種科學(xué)方法稱做“轉(zhuǎn)換法”。利用這種方法制作的儀器象：溫度計、彈簧測力計、壓強(qiáng)計等。8、力的三要素思考：同學(xué)們回顧一下，打臺球的情況。.觀察：用不同的力度（大小）打一個臺球時力越大球移動的越遠(yuǎn)。說明：力的大小影響力的作

5、用效果。觀察：用同樣的力度（大小）不同的方向打一個臺球時臺球移動的方向不一樣。說明：力的方向影響力的作用效果。觀察：用同樣的大小的力推門，每次手的位置離門軸遠(yuǎn)近不同。手在不同位置時施力的不同效果。說明：力的作用點影響力的作用效果。力的三要素：力的大小、方向、作用點注：作用在物體上的力，如果大小、方向、作用點不同，作用效果就不同，它們都影響力的作用效果。9、力的表示法：力的示意圖：用一根帶箭頭的線段把力的大小、方向、作用點表示出來,如果沒有大小,可不表示,在同一個圖中,力越大,線段應(yīng)越長具體的畫法是： (1) 確定受力物體和力的作用點和力的方向； (2) 從力的作用點按力的方向畫一個線段。用箭頭

6、表示力的方向。 (3) 力大畫的線段要長一些，或有數(shù)量和單位來表示力的大小。若在同一個圖中有幾個力，則力越大，線段應(yīng)越長。有時也可以在力的示意圖標(biāo)出力的大小。 例3：下列關(guān)于力的說法中錯誤的是：（ ）a：相互接觸的物體在產(chǎn)生力的作用。b：小孩用力推墻的同時也受到墻對小孩的推力。c：人坐在軟沙發(fā)上，可發(fā)現(xiàn)沙發(fā)凹下去，說明了力的作用可以改變物體的形狀。d：足球運(yùn)動員用頭頂球，球的運(yùn)動方向改變，表明力的作用可以改變物體的運(yùn)動狀態(tài)。例4：下列敘述的幾對力中，屬于相互作用力的是（）a：人推墻的力和鞋受到的阻力。b：墻對人的力和人受到的向后的力。c：人推墻的力和墻受到的力。d：人推墻的力和墻對人的力。習(xí)題

7、精煉：1.下列現(xiàn)象中，物體運(yùn)動狀態(tài)沒有發(fā)生變化的是 a、汽車啟動 b、汽車急剎車c、降落傘從空中勻速降落 d、汽車勻速轉(zhuǎn)彎2.用繩子系著水桶，手拿著繩子從井中向上提水，此時水桶受到向上的力，這個力的施力物體是（ ）a、手 b、繩子 c、地球 d、水3、在打雞蛋時，為什么雞蛋磕在碗邊，雞蛋會碎？4.如圖所示，運(yùn)動員用網(wǎng)拍擊球時，球和網(wǎng)拍都變了形，這表明兩點：一是 ；同時 此外，網(wǎng)拍擊球過程中，球的運(yùn)動方向和速度發(fā)生了變化，表明力還使物體的 發(fā)生改變 5.電燈對電線的拉力，施力物體是_，受力物體是_.6.俗話說“一個巴掌拍不響”，這句話所包含的物理道理是_ _；兩手用力鼓掌，雙手都會感到疼，說明力

8、的作用是_ _. 7.觀察下面兩幅圖，說出跟物理有關(guān)的知識：(1)_ (2)_ _8.下列事例中，不能說明“物體間力的作用是相互的”是（ ）a、游泳時，手和腳向后劃水，人就能前進(jìn)b、將鉛球從手中拋出去c、用手拉橡皮筋，覺得橡皮筋也在拉手d、跳高時用力向下蹬地，人就能夠向上跳起9如圖，鋼條下端固定現(xiàn)分別用不同的力去推它，發(fā)生a，b、c、d四種形變，f1=f2=f3f4，則 (1)能說明力的作用效果跟力的大小有關(guān)的是圖 和 (2)能說明力的作用效果跟力的作用點有關(guān)的圖是 和 (3)能說明力的作用效果跟力的方向有關(guān)的圖是 和 10.畫出電燈對電線的拉力。 11、駕駛員駕駛拖拉機(jī)耕地時，對于犁耕土地的

9、力，施力物體是 （ ）a 犁 b 拖拉機(jī)c 泥土 d 駕駛員12、電燈懸掛在電線下端,電線懸掛在天花板上,對于電燈所受的拉力,施力物體是（ ）a.只有地球 b.只有電線 c.地球和電線 d.電線和天花板13、在湖中劃船時,使船前進(jìn)的動力是（ ）a.人對船的推力 b.漿對水的推力 c.水直接對船的推力 d.水對漿的推力第2節(jié) 重力思考：什么是重力？ 熟了的蘋果總是向地面下落？ 衣服上的水總是落到地面上？瀑布下落，蘋果落地等，都是由于受到地球的吸引而下落。月亮繞地球轉(zhuǎn)動。由于地球與月亮之間有相互吸引的力，這個力與地球吸引蘋果，使蘋果下落的力是一樣的?？茖W(xué)家研究成果：宇宙任何兩個物體（大：天體，?。?/p>

10、灰塵）都存在相互吸引的力。引入“萬有引力”。地球?qū)Φ孛娓浇矬w的萬有引力，舉例：吊燈把吊繩拉緊；臺燈壓著桌面等。1、重力（g）：由于地球的吸引而使物體受到的力。重力的大小又叫重量。提問：重力的大小跟什么因素有關(guān)呢？演示實驗：探究重力的大小跟什么因素有關(guān)？猜想：重力的大小跟質(zhì)量有關(guān)。理由是用手提起質(zhì)量大的物體，所用的提力大，根據(jù)二力平衡，重力就大。實驗過程：分別測出一組質(zhì)量及重力的數(shù)值，并描點繪圖。由實驗結(jié)果可知：重力跟質(zhì)量成正比，比值是一個常數(shù)約9.n/。用g表示，即g=9.8n/。表示的物理意義是1的物體受到和重力是9.8n。2、重力的公式g=mg其中g(shù)表示重力（n），m表示質(zhì)量（），g是9

11、.8n/，粗略計算：g取10n/3、重力的方向：豎直向下。應(yīng)用：重垂線通常我們把與重力方向一致的線叫重錘線。重錘線在生活和生產(chǎn)上有著廣泛的應(yīng)用。如圖所示，利用重錘線可以檢查墻砌得是否豎直；還可以測桌面是否水平。4、重心：重力在物體上的作用點叫重心質(zhì)地均勻，外形規(guī)則物體的重心在物體的幾何中心。習(xí)題精煉：1、物體由于叫重力。2、重力的施力物體是 ，重力的方向是，作用點在物體的 。重力的大小可用_ _ 來測量;物體所受重力跟質(zhì)量的關(guān)系是。3、9.8牛千克表示： 4、玩具“不倒翁”被扳倒后會自動立起來的奧妙( )a 重力太小，忽略不計b 中心太低，不易傾倒c 中心的方向總是豎直向下的d 里面有自動升降

12、的裝置5、稻子被壓彎了腰，是由于水稻受到 力的作用，這個力的施力物體是 6、一個西瓜的質(zhì)量是3.5kg，它受到的重力多大？第3節(jié) 彈力思考：什么是彈力？日常生活中，用力壓尺子，尺子發(fā)生形變，撤去壓力后恢復(fù)原狀，把橡皮筋拉長，橡皮筋變長，松手后，橡皮筋恢復(fù)原狀。1、彈性：物體受力發(fā)生形變，不受力恢復(fù)原來形狀的特性。2、塑性：物體受力發(fā)生形變，不受力不能恢復(fù)原來形狀的特性。彈簧的彈性有一定的限度，超過這個限度，物體不能恢復(fù)原狀。3、彈力：物體由于發(fā)生彈性形變而產(chǎn)生的力。4、彈力產(chǎn)生的條件：物體發(fā)生彈性形變。5、彈簧測力計的制作原理：在彈性限度內(nèi)，彈簧的伸長跟受到的拉力成正比。實驗：自制彈簧測力計。

13、彈簧的一端掛在木板上，另一端掛不同重力的鉤碼，分別作標(biāo)記。提問：為什么彈簧稱的刻度是均勻的？使用測力計時應(yīng)注意的事項：（1）所測的力不能超出測力計的測量范圍；（2）使用前調(diào)整指針對準(zhǔn)零刻線；（3）認(rèn)清測力計的最大測量值和最小測量值；（4）測力計測重力時不能傾斜放置。習(xí)題精煉：1、在物理實驗室中，測量力的儀器或工具是( ) a.彈簧秤 b.天平 c.量筒 d.壓強(qiáng)計 2、關(guān)于彈簧測力計上零刻度的意義，下列說法中錯誤的是 ( ) a.彈簧的長度為零 b彈簧的伸長為零 c.彈簧所受的拉力為零 d指針的初始位置 3、下列關(guān)于彈力的說法中，正確的是 ( ) a相互接觸的物體之間一定存在彈力作用 b只有受

14、彈簧作用的物體才受到彈力作用c只有相互接觸并發(fā)生形變的物體間才存在彈力作用 d彈簧的彈力總是跟彈簧的長度成正比 4、某同學(xué)在用彈簧測力計測量力前，發(fā)現(xiàn)指針指在0.1n的位置上，為了使測量準(zhǔn)確，他提出了以下調(diào)整方法，其中正確的是( ) a. 必須將指針撥到“0”刻度線，若不能使指針對準(zhǔn)零刻度線，該彈簧測力計已不能使用b仍可以直接測出拉力，然后再減去0.1n c仍可以直接測出拉力數(shù)值，因為實驗時誤差不可避免d. 必須換一個彈簧測力計測量，該彈簧測力計已不能使用 5、甲、乙兩位同學(xué)分別用20 n的力拉一個彈簧測力計的兩端，關(guān)于測力計示數(shù)說法正確的是a.20 n b.40 n c.0 n d.無法確定

15、 6、在彈簧的彈性限度內(nèi)，彈簧下端掛300克物體時長15厘米，若改掛500克的物體， 則彈簧長：、25厘米;、20厘米; 、17厘米;、不能確定.7、某中學(xué)九年級（1）班物理課外興趣小組在收集有關(guān)質(zhì)量的測量工具時整理信息如下：（一）天平和臺秤可以測出物體的質(zhì)量，用某天平測物體的質(zhì)量時，右盤的砝碼及游碼如圖7 所示，則左盤物體的質(zhì)量為_g。 （二）用彈簧秤進(jìn)行測量也可得出物體的質(zhì)量。某次測量時彈簧秤的示數(shù)如圖8所示。若g取10n / kg，則物體的質(zhì)量為_kg。 winger tuivasa-sheck, who scored two tries in the kiwis 20-18 semi-

16、final win over england, has been passed fit after a lower-leg injury, while slater has been named at full-back but is still recovering from a knee injury aggravated against usa.both sides boast 100% records heading into the encounter but australia have not conceded a try since josh charnleys effort

17、in their first pool match against england on the opening day.aussie winger jarryd hayne is the competitions top try scorer with nine, closely followed by tuivasa-sheck with eight.but it is recently named rugby league international federation player of the year sonny bill williams who has attracted t

18、he most interest in the tournament so far.the kiwi - with a tournament high 17 offloads - has the chance of becoming the first player to win the world cup in both rugby league and rugby union after triumphing with the all blacks in 2011.id give every award back in a heartbeat just to get across the

19、line this weekend, said williams.the (lack of) air up there watch mcayman islands-based webb, the head of fifas anti-racism taskforce, is in london for the football associations 150th anniversary celebrations and will attend citys premier league match at chelsea on sunday.i am going to be at the mat

20、ch tomorrow and i have asked to meet yaya toure, he told bbc sport.for me its about how he felt and i would like to speak to him first to find out what his experience was.uefa hasopened disciplinary proceedings against cskafor the racist behaviour of their fans duringcitys 2-1 win.michel platini, pr

21、esident of european footballs governing body, has also ordered an immediate investigation into the referees actions.cska said they were surprised and disappointed by toures complaint. in a statement the russian side added: we found no racist insults from fans of cska. baumgartner the disappointing n

22、ews: mission aborted.the supersonic descent could happen as early as sunda.the weather plays an important role in this mission. starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. the balloon, with capsule attach

23、ed, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. it will climb higher than the tip of mount everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into t

24、he stratosphere. as he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.the balloon will slowly drift to the edge of space at 120,000 feet ( then, i would assume, he will slowly step out onto something resembling an olympic diving platform.they blew it in 2008 when

25、 they got caught cold in the final and they will not make the same mistake against the kiwis in manchester.five years ago they cruised through to the final and so far history has repeated itself here - the last try they conceded was scored by englands josh charnley in the opening game of the tournam

26、ent.that could be classed as a weakness, a team under-cooked - but i have been impressed by the kangaroos focus in their games since then.they have been concentrating on the sort of stuff that wins you tough, even contests - strong defence, especially on their own goal-line, completing sets and a go

27、od kick-chase. theyve been great at all the unglamorous stuff that often goes unnoticed in the stands but not by your team-mates.it is as though their entire tournament has been preparation for the final.in johnathan thurston, cooper cronk, cameron smith and either billy slater or greg inglis at ful

28、l-back they have a spine that is unmatched in rugby league. they have played in so many high-pressure games - a priceless asset going into saturday.the kiwis are a lot less experienced but winning a dramatic match like their semi-final against england will do wonders for their confidence.they defeat

29、ed australia in the four nations final in 2010 and the last world cup, and know they can rise to the big occasion.winger tuivasa-sheck, who scored two tries in the kiwis 20-18 semi-final win over england, has been passed fit after a lower-leg injury, while slater has been named at full-back but is s

30、till recovering from a knee injury aggravated against usa.both sides boast 100% records heading into the encounter but australia have not conceded a try since josh charnleys effort in their first pool match against england on the opening day.aussie winger jarryd hayne is the competitions top try sco

31、rer with nine, closely followed by tuivasa-sheck with eight.but it is recently named rugby league international federation player of the year sonny bill williams who has attracted the most interest in the tournament so far.the kiwi - with a tournament high 17 offloads - has the chance of becoming th

32、e first player to win the world cup in both rugby league and rugby union after triumphing with the all blacks in 2011.id give every award back in a heartbeat just to get across the line this weekend, said williams.the (lack of) air up there watch mcayman islands-based webb, the head of fifas anti-ra

33、cism taskforce, is in london for the football associations 150th anniversary celebrations and will attend citys premier league match at chelsea on sunday.i am going to be at the match tomorrow and i have asked to meet yaya toure, he told bbc sport.for me its about how he felt and i would like to spe

34、ak to him first to find out what his experience was.uefa hasopened disciplinary proceedings against cskafor the racist behaviour of their fans duringcitys 2-1 win.michel platini, president of european footballs governing body, has also ordered an immediate investigation into the referees actions.csk

35、a said they were surprised and disappointed by toures complaint. in a statement the russian side added: we found no racist insults from fans of cska. baumgartner the disappointing news: mission aborted.the supersonic descent could happen as early as sunda.the weather plays an important role in this

36、mission. starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. the balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. it will c

37、limb higher than the tip of mount everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. as he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.the balloon will slowly drift to the edge of space at 120,000 feet ( then, i would assume, he will slowly step out onto something resembling an olympic diving platform.they blew it in 2008 when they got caught cold in the final and they will not make