最新人教版八年级下册物理《力》完整讲义

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1、专业好文档第七章 力第1节 力1、 生活中存在的力 拉弓射箭 运动员举起杠铃 运动员投篮 压路机压路思考：什么是力？人 弓推土机 土运动员 杠铃物 体 物体2、力的概念：力是物体对物体的作用 受力物体： 施力物体： 3、力产生的条件：必须有两个或两个以上的物体。物体间必须有相互作用（可以不接触）。4、力的性质：物体间力的作用是相互的（相互作用力在任何情况下都是大小相等，方向相反，作用在不同物体上）。两物体相互作用时，施力物体同时也是受力物体，反之，受力物体同时也是施力物体。思考：力的作用效果有哪些？ 用力扳竹子 小孩盘腿坐在木板上 骆驼在沙地上踩出脚印 足球运动中相互传球 守门员守球5、力的作

2、用效果：力可以改变物体的运动状态 力可以改变物体的形状说明：物体的运动状态是否改变一般指：物体的运动快慢是否改变（速度大小的改变）和物体的运动方向是否改变例1：在跳板跳水这个运动项目中运动员对跳板施力的同时，也受到跳板对他的作用力但这两个力的作用效果是不一样的。前者要改变了跳板的 ，后者主要改变了运动员的 例2：下列实例中，力的作用下使物体的形状发生变化的是：（ ）A：紧急刹车。B：骑自行车加速前进。C：做直线运动的足球碰到球员后，运动方向发生改变。D：两手用力扳竹条，使其弯曲。点拨：理解力的概念应把握两条：1、力就是作用，力现象中的物体间推、拉、提、压、排斥、吸引等都叫力的作用。2、力不能离

3、开物体而存在，有力至少有两个物体，一个是施力物体，另外一个是受力物体。运动状态的理解：1、由于物体的运动和静止是相对的，因此我们把静止的当作是一个特殊的运动状态。2、一个物体保持静止或做匀速直线运动，我们就说它的运动状态不变，否则我们就说它的运动状态变了。6、力的单位：国际单位制中力的单位是牛顿简称牛，用N 表示。力的感性认识：拿两个鸡蛋所用的力大约1N。7、力的测量：测力计：测量力的大小的工具。分类：弹簧测力计、握力计。弹簧测力计：A、原理：在弹性限度内，弹簧的伸长与所受的拉力成正比。B、使用方法：(1)要检查指针是否指在零刻度，如果不是，则要调零；(2)认清最小刻度和测量范围；(3)轻拉秤

4、钩几次，看每次松手后，指针是否回到零刻度，(4)测量时弹簧测力计内弹簧的轴线与所测力的方向一致；(5)观察读数时，视线必须与刻度盘垂直。(6)测量力时不能超过弹簧测力计的量程。C、注意事项：加在弹簧测力计上的力不许超过它的最大量程。D、物理实验中,有些物理量的大小是不宜直接观察的,但它变化时引起其他物理量的变化却容易观察,用容易观察的量显示不宜观察的量,是制作测量仪器的一种思路。这种科学方法称做“转换法”。利用这种方法制作的仪器象：温度计、弹簧测力计、压强计等。8、力的三要素思考：同学们回顾一下，打台球的情况。.观察：用不同的力度（大小）打一个台球时力越大球移动的越远。说明：力的大小影响力的作

5、用效果。观察：用同样的力度（大小）不同的方向打一个台球时台球移动的方向不一样。说明：力的方向影响力的作用效果。观察：用同样的大小的力推门，每次手的位置离门轴远近不同。手在不同位置时施力的不同效果。说明：力的作用点影响力的作用效果。力的三要素：力的大小、方向、作用点注：作用在物体上的力，如果大小、方向、作用点不同，作用效果就不同，它们都影响力的作用效果。9、力的表示法：力的示意图：用一根带箭头的线段把力的大小、方向、作用点表示出来,如果没有大小,可不表示,在同一个图中,力越大,线段应越长具体的画法是： (1) 确定受力物体和力的作用点和力的方向； (2) 从力的作用点按力的方向画一个线段。用箭头

6、表示力的方向。 (3) 力大画的线段要长一些，或有数量和单位来表示力的大小。若在同一个图中有几个力，则力越大，线段应越长。有时也可以在力的示意图标出力的大小。 例3：下列关于力的说法中错误的是：（ ）A：相互接触的物体在产生力的作用。B：小孩用力推墙的同时也受到墙对小孩的推力。C：人坐在软沙发上，可发现沙发凹下去，说明了力的作用可以改变物体的形状。D：足球运动员用头顶球，球的运动方向改变，表明力的作用可以改变物体的运动状态。例4：下列叙述的几对力中，属于相互作用力的是（）A：人推墙的力和鞋受到的阻力。B：墙对人的力和人受到的向后的力。C：人推墙的力和墙受到的力。D：人推墙的力和墙对人的力。习题

7、精炼：1.下列现象中，物体运动状态没有发生变化的是 A、汽车启动 B、汽车急刹车C、降落伞从空中匀速降落 D、汽车匀速转弯2.用绳子系着水桶，手拿着绳子从井中向上提水，此时水桶受到向上的力，这个力的施力物体是（ ）A、手 B、绳子 C、地球 D、水3、在打鸡蛋时，为什么鸡蛋磕在碗边，鸡蛋会碎？4.如图所示，运动员用网拍击球时，球和网拍都变了形，这表明两点：一是 ；同时 此外，网拍击球过程中，球的运动方向和速度发生了变化，表明力还使物体的 发生改变 5.电灯对电线的拉力，施力物体是_，受力物体是_.6.俗话说“一个巴掌拍不响”，这句话所包含的物理道理是_ _；两手用力鼓掌，双手都会感到疼，说明力

8、的作用是_ _. 7.观察下面两幅图，说出跟物理有关的知识：(1)_ (2)_ _8.下列事例中，不能说明“物体间力的作用是相互的”是（ ）A、游泳时，手和脚向后划水，人就能前进B、将铅球从手中抛出去C、用手拉橡皮筋，觉得橡皮筋也在拉手D、跳高时用力向下蹬地，人就能够向上跳起9如图，钢条下端固定现分别用不同的力去推它，发生a，b、c、d四种形变，F1=F2=F3F4，则 (1)能说明力的作用效果跟力的大小有关的是图 和 (2)能说明力的作用效果跟力的作用点有关的图是 和 (3)能说明力的作用效果跟力的方向有关的图是 和 10.画出电灯对电线的拉力。 11、驾驶员驾驶拖拉机耕地时，对于犁耕土地的

9、力，施力物体是 （ ）A 犁 B 拖拉机C 泥土 D 驾驶员12、电灯悬挂在电线下端,电线悬挂在天花板上,对于电灯所受的拉力,施力物体是（ ）A.只有地球 B.只有电线 C.地球和电线 D.电线和天花板13、在湖中划船时,使船前进的动力是（ ）A.人对船的推力 B.浆对水的推力 C.水直接对船的推力 D.水对浆的推力第2节 重力思考：什么是重力？ 熟了的苹果总是向地面下落？ 衣服上的水总是落到地面上？瀑布下落，苹果落地等，都是由于受到地球的吸引而下落。月亮绕地球转动。由于地球与月亮之间有相互吸引的力，这个力与地球吸引苹果，使苹果下落的力是一样的。科学家研究成果：宇宙任何两个物体（大：天体，小：

10、灰尘）都存在相互吸引的力。引入“万有引力”。地球对地面附近物体的万有引力，举例：吊灯把吊绳拉紧；台灯压着桌面等。1、重力（G）：由于地球的吸引而使物体受到的力。重力的大小又叫重量。提问：重力的大小跟什么因素有关呢？演示实验：探究重力的大小跟什么因素有关？猜想：重力的大小跟质量有关。理由是用手提起质量大的物体，所用的提力大，根据二力平衡，重力就大。实验过程：分别测出一组质量及重力的数值，并描点绘图。由实验结果可知：重力跟质量成正比，比值是一个常数约9.N/。用g表示，即g=9.8N/。表示的物理意义是1的物体受到和重力是9.8N。2、重力的公式G=mg其中G表示重力（N），m表示质量（），g是9

11、.8N/，粗略计算：g取10N/3、重力的方向：竖直向下。应用：重垂线通常我们把与重力方向一致的线叫重锤线。重锤线在生活和生产上有着广泛的应用。如图所示，利用重锤线可以检查墙砌得是否竖直；还可以测桌面是否水平。4、重心：重力在物体上的作用点叫重心质地均匀，外形规则物体的重心在物体的几何中心。习题精炼：1、物体由于叫重力。2、重力的施力物体是 ，重力的方向是，作用点在物体的 。重力的大小可用_ _ 来测量;物体所受重力跟质量的关系是。3、9.8牛千克表示： 4、玩具“不倒翁”被扳倒后会自动立起来的奥妙( )A 重力太小，忽略不计B 中心太低，不易倾倒C 中心的方向总是竖直向下的D 里面有自动升降

12、的装置5、稻子被压弯了腰，是由于水稻受到 力的作用，这个力的施力物体是 6、一个西瓜的质量是3.5Kg，它受到的重力多大？第3节 弹力思考：什么是弹力？日常生活中，用力压尺子，尺子发生形变，撤去压力后恢复原状，把橡皮筋拉长，橡皮筋变长，松手后，橡皮筋恢复原状。1、弹性：物体受力发生形变，不受力恢复原来形状的特性。2、塑性：物体受力发生形变，不受力不能恢复原来形状的特性。弹簧的弹性有一定的限度，超过这个限度，物体不能恢复原状。3、弹力：物体由于发生弹性形变而产生的力。4、弹力产生的条件：物体发生弹性形变。5、弹簧测力计的制作原理：在弹性限度内，弹簧的伸长跟受到的拉力成正比。实验：自制弹簧测力计。

13、弹簧的一端挂在木板上，另一端挂不同重力的钩码，分别作标记。提问：为什么弹簧称的刻度是均匀的？使用测力计时应注意的事项：（1）所测的力不能超出测力计的测量范围；（2）使用前调整指针对准零刻线；（3）认清测力计的最大测量值和最小测量值；（4）测力计测重力时不能倾斜放置。习题精炼：1、在物理实验室中，测量力的仪器或工具是( ) A.弹簧秤 B.天平 C.量筒 D.压强计 2、关于弹簧测力计上零刻度的意义，下列说法中错误的是 ( ) A.弹簧的长度为零 B弹簧的伸长为零 C.弹簧所受的拉力为零 D指针的初始位置 3、下列关于弹力的说法中，正确的是 ( ) A相互接触的物体之间一定存在弹力作用 B只有受

14、弹簧作用的物体才受到弹力作用C只有相互接触并发生形变的物体间才存在弹力作用 D弹簧的弹力总是跟弹簧的长度成正比 4、某同学在用弹簧测力计测量力前，发现指针指在0.1N的位置上，为了使测量准确，他提出了以下调整方法，其中正确的是( ) A. 必须将指针拨到“0”刻度线，若不能使指针对准零刻度线，该弹簧测力计已不能使用B仍可以直接测出拉力，然后再减去0.1N C仍可以直接测出拉力数值，因为实验时误差不可避免D. 必须换一个弹簧测力计测量，该弹簧测力计已不能使用 5、甲、乙两位同学分别用20 N的力拉一个弹簧测力计的两端，关于测力计示数说法正确的是A.20 N B.40 N C.0 N D.无法确定

15、 6、在弹簧的弹性限度内，弹簧下端挂300克物体时长15厘米，若改挂500克的物体， 则弹簧长：、25厘米;、20厘米; 、17厘米;、不能确定.7、某中学九年级（1）班物理课外兴趣小组在收集有关质量的测量工具时整理信息如下：（一）天平和台秤可以测出物体的质量，用某天平测物体的质量时，右盘的砝码及游码如图7 所示，则左盘物体的质量为_g。 （二）用弹簧秤进行测量也可得出物体的质量。某次测量时弹簧秤的示数如图8所示。若g取10N / kg，则物体的质量为_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

38、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 the same mistake against the Kiwis in Manchester.Five years a

39、go 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 tournament.That could be classed as a weakness, a team under-cooked - but I have been impressed by the Kangaroos focus in thei

40、r 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 good kick-chase. Theyve been great at all the unglamorous stuff that often goes unnoticed in the stands but not by your t

41、eam-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 full-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 defeated Australia in the Four Nations final in 2010 and the last World Cup, and know they can rise to the big occasion.13