小班講義高一物理第8講牛頓運(yùn)動(dòng)定律的應(yīng)用1—瞬時(shí)問題

1、龍 文 教 育精品小班課程輔導(dǎo)講義講義編號(hào) 2013秋季 輔導(dǎo)科目： 物理 年級(jí)： 高一課 題第8講：牛頓運(yùn)動(dòng)定律的應(yīng)用1瞬時(shí)問題、彈簧問題教學(xué)目標(biāo)1、理解牛頓運(yùn)動(dòng)定律的瞬時(shí)性，矢量性等。2、會(huì)運(yùn)用牛頓運(yùn)動(dòng)定律解決物體與彈簧相連時(shí)的運(yùn)動(dòng)過程分析。教學(xué)重點(diǎn)、難點(diǎn)1掌握彈簧彈力和剛體彈力特點(diǎn)，會(huì)求任意時(shí)刻的力與加速度；2掌握彈簧彈力的特點(diǎn)，能合理應(yīng)用牛頓運(yùn)動(dòng)定律解決有關(guān)彈簧問題，會(huì)討論有彈簧的情況下力和加速度的變化，會(huì)討論有彈簧的情況下物體的運(yùn)動(dòng)情況教學(xué)內(nèi)容活動(dòng)一、 完成問題，熟悉牛頓第二定律的應(yīng)用例題1.如圖所示，一輕質(zhì)彈簧一端與墻相連，另一端與一物體接觸（不相連），當(dāng)彈簧在o點(diǎn)位置時(shí)彈簧沒有形

2、變，現(xiàn)用力將物體壓縮至a點(diǎn)，然后放手。物體向右運(yùn)動(dòng)至c點(diǎn)而靜止， 圖中ab段光滑，bc段粗糙?；卮鹣铝袉栴}1. 分析物塊在ao,ob,bc段各做什么運(yùn)動(dòng)？2. 試用v-t圖像表示物塊的運(yùn)動(dòng)。3. 物塊在哪個(gè)位置加速度最大？若物塊質(zhì)量為m，勁度系數(shù)為k，oa長為x，則物體具有的最大加速度為？活動(dòng)二、思考下列問題，學(xué)會(huì)分析涉及彈簧的物體運(yùn)動(dòng)過程例題2。一物體從某一高度由靜止開始自由落下，落在直立固定的輕彈簧上，如圖所示，在a點(diǎn)物體開始與彈簧接觸，一直到c點(diǎn)時(shí)物體的速度為零。分析運(yùn)動(dòng)過程，回答問題（1）小球從初始位置o到c的運(yùn)動(dòng)過程中，可以分為哪幾個(gè)階段？（2）分析各個(gè)階段的受力，并寫出合力的表達(dá)式

3、？（3）定性分析物體從o到c的運(yùn)動(dòng)情況？（4）你能用v-t圖像形象的表示出物體從o到c的運(yùn)動(dòng)過程嗎？（5）定量計(jì)算，若物體的質(zhì)量為1kg，彈簧的勁度系數(shù)k=500n/m，ac的距離是5cm，求物體速度最大時(shí)彈簧的形變量是多少？及物體到達(dá)c點(diǎn)時(shí)的加速度？活動(dòng)三、適度拓展，進(jìn)一步理解掌握涉及彈簧的物體在運(yùn)動(dòng)過程中的定量分析例題3.如圖所示，豎直放置的勁度系數(shù)的輕彈簧上有一質(zhì)量不計(jì)的輕盤，盤內(nèi)放著一個(gè)質(zhì)量的物體，一開始m處于靜止?fàn)顟B(tài)，現(xiàn)給物體施加一個(gè)豎直向上的力f，使物體從靜止開始向上一直做勻加速直線運(yùn)動(dòng)。已知開始內(nèi)f是變力，在后f是恒力，取問題討論（1）物塊與輕盤什么時(shí)候分離？物塊在與輕盤分離之前

4、受到哪些力？分析分離前拉力f應(yīng)該如何變化？（2）在開始的0.2s內(nèi)物塊上升的距離是多少？上升階段加速度是多大（3）拉力f在什么時(shí)候最小，什么時(shí)候最大？求出f最小值和最大值經(jīng)典例題圖3-5-1例1：如圖3-5-1所示，一根輕質(zhì)彈簧上端固定，下端掛一質(zhì)量為m的平托盤，在盤中有一質(zhì)量為m的物體，當(dāng)盤靜止時(shí)，彈簧的長度比自然長度伸長了l，現(xiàn)向下拉盤使彈簧再伸長l后停止，然后松手，設(shè)彈簧總處于彈性限度內(nèi)，則剛松開手時(shí)，盤對(duì)物體的支持力大小等于多少？分析與解：松手后，求支持力研究m，受力情況與運(yùn)動(dòng)情況分析如圖3-5-2所示，則 mgna（m+m）gfa圖3-5-2圖3-5-3求支持力也就是求加速度，而m與

5、m的加速度相同，所以我們可以研究整體，受力情況與運(yùn)動(dòng)情況分析如圖3-5-3所示，其中彈簧彈力變化需要形變發(fā)生變化，形變發(fā)生變化需要時(shí)間，在瞬間還可認(rèn)為沒變，形變量為所以，將開始平衡時(shí)的條件代入，得， abco1o2圖3-5-4例2：質(zhì)量為3m的箱子c，頂部懸掛質(zhì)量為2m的小球b，小球b的下方通過一輕彈簧與質(zhì)量仍為m的小球a相連，箱子c用輕繩o1o2懸于天花板上而處于平衡狀態(tài)，如圖3-5-4所示現(xiàn)剪斷輕繩o1o2，在剪斷輕繩的瞬間，小球a、b和箱子c的加速度分別為多少？分析與解：研究a球，受到重力和彈簧的彈力作用，因?yàn)閺椈蓮椓Φ拇笮∨c彈簧的形變有關(guān)，形變需要時(shí)間，在瞬間（一個(gè)相當(dāng)短的時(shí)間），可以

6、認(rèn)為彈簧彈力沒變，大小等于a物體的重力mg所以，a球的加速度為零研究b球，受到重力、彈簧的彈力（瞬間大小仍為mg），但繩子中的拉力在瞬間發(fā)生變化了繩中是否有彈力，有的話變?yōu)槎嗌?？不清楚所以，我們可以假設(shè)沒有拉力，b的受力情況與運(yùn)動(dòng)情況分析如圖3-5-5所示，2mgfabb圖3-5-5則而c只受到重力，所以2mg+3mgfabcb、c圖3-5-6，所以假設(shè)不成立，繩子將拉緊，b、c的加速度相同研究b、c，受力情況與運(yùn)動(dòng)情況分析如圖3-5-6所示，則圖3-5-7例3：兩個(gè)質(zhì)量不計(jì)的彈簧將一金屬塊支在箱子的上頂板與下底板之間，箱只能沿豎直方向運(yùn)動(dòng)，如圖3-5-7所示兩彈簧原長均為0.80m，勁度系數(shù)

7、均為60n/m當(dāng)箱以a=2.0m/s2的加速度勻減速上升時(shí)，上彈簧的長度為0.70m，下彈簧的長度為0.60m現(xiàn)若上頂板彈力是下底板彈力的四分之一，試判斷箱的運(yùn)動(dòng)情況（g=10m/s2）mgf1f2a圖3-5-8分析與解：設(shè)原長l=0.80m，l1=0.70m，l2=0.60m當(dāng)箱以a=2.0m/s2的加速度勻減速上升時(shí)，研究箱受力情況與運(yùn)動(dòng)情況分析如圖3-5-8所示，則當(dāng)上頂板彈力是下底板彈力的四分之一，研究箱，由于題目中講的是彈力，不知是拉力還是壓力，所以，要分情況討論，設(shè)上彈簧的長為l3，下彈簧的長度為l4，則l3+ l4= l1+ l2=1.30m（1）假設(shè)上彈簧拉長，下彈簧壓縮，則依

8、題意有 l3=0.90m，l4=0.40m（假設(shè)成立）箱子的加速度為（2）假設(shè)上彈簧拉長，下彈簧拉長，則依題意不可能（3）假設(shè)上彈簧壓縮，下彈簧拉長，則依題意有 l3=0.90m，l4=0.40m（假設(shè)不成立）（4）假設(shè)上彈簧壓縮，下彈簧壓縮，則依題意有 l3=0.76m，l4=0.56m（假設(shè)成立）箱子的加速度為所以，箱子可能的運(yùn)動(dòng)是：箱子向上加速或者是向下減速，加速度的大小為或者afb圖3-5-9例4：如圖3-5-9所示，物體a靜止在臺(tái)秤的秤盤b上，a的質(zhì)量ma=10.5kg，b的質(zhì)量mb=1.5kg，彈簧的質(zhì)量忽略不計(jì)，彈簧的勁度系數(shù)k=800n/m現(xiàn)給a施加一個(gè)豎直向上的力f，使它向上

9、做勻加速直線運(yùn)動(dòng)，已知力f在t=0.2s內(nèi)是變力，在0.2s后是恒力.求此過程中f的最大值與最小值（g=10m/s2）分析與解：研究a、b整體，受力情況與運(yùn)動(dòng)情況分析如圖3-5-10（甲）所示，則由（1）式可知，當(dāng)向上運(yùn)動(dòng)時(shí)，x減小，f增加（ma+ mb）gffamagnfambgfa圖3-5-10甲乙丙ab再研究a物體，受力情況與運(yùn)動(dòng)情況分析如圖3-5-10（乙）所示，則由（2）式可知，隨著f的增加（x減?。琻減小，當(dāng)n減小到零后（n保持為零，不會(huì)小于零），f就不變了所以，開始時(shí)，x最大，f最小，最后，支持力等于零時(shí)，x最小，f最大研究整體，不加f時(shí)，加速度為零，則由（1）式得研究脫離時(shí)的

10、b盤，此時(shí)f最大，受力情況與運(yùn)動(dòng)情況分析如圖3-5-10（丙）所示，則且解（3）（4）式得 ，a=6m/s2，將n=0、a=6m/s2代入（2），則得f的最大值研究 t =0時(shí)，（1）式中x即為x1，此時(shí)，f為最小值【思路點(diǎn)撥】一般產(chǎn)生彈力的物體可分為兩類，一類是彈簧（尤其是軟件包彈簧）它的形變發(fā)生改變的過程需時(shí)較長在瞬間（時(shí)間很短）內(nèi)形變量可認(rèn)為是不變的；另一類是鋼絲（包括細(xì)絲）等屬于剛體，它的形變發(fā)生改變過程極短（比瞬間還短），在極短的時(shí)間內(nèi)彈力能發(fā)生很大的變化所以，遇到研究彈力變化的問題時(shí)，先要把研究的對(duì)象等效為軟彈簧模型或剛體模型，然后再進(jìn)行分析計(jì)算在名題精析例1中，松手的瞬間，彈簧中

11、的彈力變化很小，所以被忽略了，而盤與物體間的彈力變化較大，不能被忽略例2中，同樣如此，彈簧中的彈力變化被忽略了，但繩中彈力的變化是要考慮的，在例1、例2中我們求出了瞬時(shí)力和瞬時(shí)加速度在例3中，我們知道彈簧中的彈力可能拉力，也可能是壓力，題中沒說清的話，就要討論，這樣才不至于漏解，同時(shí)值得注意的是，在例3中，2根彈簧的實(shí)際長度之間是有關(guān)系的，這個(gè)關(guān)系也經(jīng)常要用到在例4中，結(jié)合彈簧彈力特點(diǎn)，討論了物體運(yùn)動(dòng)過程中物理量（力）的變化問題，從而找出了臨界值，求出了最大值與最小值 第 3 次課后作業(yè)學(xué)生姓名： 1靜止在光滑水平面上的物體受到水平恒定拉力f作用，則下列說法正確的是（ ）af剛作用的瞬間，物體

12、立即獲得速度和加速度bf剛作用的瞬間，物體立即獲得加速度，但速度仍為零c經(jīng)過一段時(shí)間，物體速度增加，加速度也增加d當(dāng)撤去外力f時(shí)，加速度立即變?yōu)榱?，物體做勻速直線運(yùn)動(dòng)圖3-5-112如圖3-5-11所示，質(zhì)量為m的框架放在水平地面上，一輕質(zhì)彈簧上端固定在框架上，一端栓著一個(gè)質(zhì)量為m的小球，小球上下振動(dòng)時(shí)，框架始終沒有跳起，當(dāng)框架對(duì)地面壓力為零的瞬間，小球的加速度大小為（ ）ag b（m-m）g/m c0 d（m+m）g/mmn30ba圖3-5-123如圖3-5-12所示，傾角為30的光滑桿上套有一個(gè)小球和兩根輕質(zhì)彈簧a、b，兩彈簧的一端各與小球相連，另一端分別用銷釘m、n固定于桿上，小球處于靜

13、止?fàn)顟B(tài)，設(shè)拔去銷釘m瞬間，小球的加速度大小為6m/s2，若不拔去銷釘m，而拔去銷釘n瞬間，小球的加速度是（g取10m/s2）（ ）a11m/s2，沿桿向上 b11m/s2，沿桿向下c1m/s2， 沿桿向上 d1m/s2， 沿桿向下abc圖3-5-134如圖3-5-13所示，小球質(zhì)量為m，被三根相同的輕質(zhì)彈簧a、b、c拉住，c豎直向下，a、b、c三者都夾120角小球平衡時(shí)a、b、c彈力大小之比為331，設(shè)重力加速度為g，則在剪斷彈簧c瞬間小球加速度的大小及方向可能為（ ）ag/2，豎直向下 bg/2，豎直向上cg/4，豎直向下 dg/4，豎直向上5如圖3-5-14所示，質(zhì)量為m的物體a放置在質(zhì)量

14、為m的物體b上，b與彈簧相連，它們一起在光滑水平面上作簡諧振動(dòng)，振動(dòng)過程中a、b之間無相對(duì)運(yùn)動(dòng)設(shè)彈簧的勁度系數(shù)為k當(dāng)物體離開平衡位置的位移為x時(shí)，a、b間摩擦力的大小等于（ ）ba圖3-5-14a0 bkx cm kx /m d m kx /(mm)圖3-5-156如圖3-5-15所示，輕質(zhì)彈簧上面固定一塊質(zhì)量不計(jì)的薄板，豎立在水平面上，在薄板上放一重物，用手將重物向下壓縮到一定程度后，突然將手撤去，則重物將被彈簧彈射出去，則在彈射過程中（重物與彈簧脫離之前）（ ）a重物受的支持力先減小后增大 b彈簧的彈力先減小后增大c重物的加速度先減小后增大 d重物先加速后減速fab圖3-5-167如圖3-

15、5-16所示，質(zhì)量相同的木塊a、b用輕繩連接靜止于光滑的水平面上，彈簧處于自然狀態(tài)現(xiàn)用水平恒力f推a，則從開始到彈簧第一次被壓縮到最短的過程中（ ）a兩木塊速度相同時(shí)，aa=ab b兩木塊速度相同時(shí)，加速度aaabc兩木塊加速度相同時(shí)，速度vavbl1l2a b l1l2圖3-5-178如圖3-5-17（a）所示，一質(zhì)量為m的物體系于長度分別為l1、l2的兩根細(xì)線上，l1的一端懸掛在天花板上，與豎直方向夾角為，l2水平拉直，物體處于平衡狀態(tài)現(xiàn)將l2線剪斷，求剪斷瞬時(shí)物體的加速度（l）下面是某同學(xué)對(duì)該題的一種解法：解：設(shè)l1線上拉力為t1，線上拉力為t2，重力為mg，物體在三力作用下保持平衡t1

16、cosmg， t1sint2， t2mgtg剪斷線的瞬間，t2突然消失，物體即在t2反方向獲得加速度因?yàn)閙g tgma，所以加速度ag tg，方向在t2反方向你認(rèn)為這個(gè)結(jié)果正確嗎？請(qǐng)對(duì)該解法作出評(píng)價(jià)并說明理由（2）若將圖3-5-17（a）中的細(xì)線l1改為長度相同、質(zhì)量不計(jì)的輕彈簧，如圖3-5-17（b）所示，其他條件不變，求解的步驟和結(jié)果與（l）完全相同，即 ag tg，你認(rèn)為這個(gè)結(jié)果正確嗎？請(qǐng)說明理由abf圖3-5-189如圖3-5-18所示，兩矩形物塊a和b疊放在豎直的彈簧上，已知ma=2m，mb=m今用一豎直向下的大小為f的力壓物塊a（未超過彈簧的彈性限度），在突然撤去力f的瞬時(shí)a對(duì)b的

17、支持力多大？ 10有三個(gè)質(zhì)量分別為5m、3m和2m的小球a、b和c，其中b球帶正電q，其余兩球不帶電用足夠長的不會(huì)伸長的絕緣線連接均置于豎直向下的勻強(qiáng)電場中，場強(qiáng)大小為e如圖3-5-19所示釋放a球，讓三球由靜止落下，下落一小段時(shí)間（球不相碰），此時(shí)三球的加速度大小分別為多少？abce圖3-5-19+q11如圖3-5-20所示，木塊a與b用一輕彈簧相連，豎直放在c板上，a、b、c的質(zhì)量分別為m、2m、3m（1）若將三者靜止放在水平地面上，當(dāng)沿水平方向迅速抽出c的瞬時(shí)，a、b的加速度分別多大？方向如何？（2）若用手托住c板，為使c板能即刻與b分離，則手至少要對(duì)c施加向下多大的力？abc圖3-5-

18、2012如圖3-5-21所示，小車放在水平面上，在水平外力的作用下沿水平向右做勻加速直線運(yùn)動(dòng)，放在小車上的兩個(gè)物體a、b的質(zhì)量分別為ma=1kg，mb=0.5kg它們與小車表面的動(dòng)摩擦因數(shù)分別是a=0.4，b=0.2，連接兩個(gè)物體的輕質(zhì)彈簧的勁度系數(shù)k=10n/m設(shè)最大靜摩擦力等于滑動(dòng)摩擦力求當(dāng)小車的加速度a=2.5m/s2時(shí)，a、b所受的摩擦力和彈簧的形變量（g=10m/s2）傳感器a傳感器bv圖3-5-22ab圖3-5-2113用如圖3-5-22所示的裝置可以測(cè)量汽車在水平路面上做勻加速直線運(yùn)動(dòng)的加速度該裝置是在矩形箱子的前、后壁上各安裝一個(gè)由力敏電阻組成的壓力傳感器用兩根相同的輕彈簧夾著

19、一個(gè)質(zhì)量為2.0kg的滑塊，滑塊可無摩擦滑動(dòng)，兩彈簧的另一端分別壓在傳感器a、b上，其壓力大小可直接從傳感器的液晶顯示屏上讀出現(xiàn)將裝置沿運(yùn)動(dòng)方向固定在汽車上，傳感器b在前，傳感器a在后汽車靜止時(shí)，傳感器a、b的示數(shù)均為10n（取g=10m/s2） （1）若示感器a的示數(shù)為14n，求此時(shí)汽車的加速度大小和方向（2）當(dāng)汽車以怎樣的加速度運(yùn)動(dòng)時(shí)，傳感器a的示數(shù)為零圖3-5-2314將金屬塊用壓縮的彈簧卡在一個(gè)矩形的箱中，如圖3-5-23所示，在箱的上頂板和下頂板安有壓力傳感器，箱可以沿直軌道運(yùn)動(dòng)當(dāng)箱以a=2m/s2的加速度做豎直向上的勻減速運(yùn)動(dòng)時(shí)，上頂板的傳感器顯示的壓力為6.0n，下頂板的傳感器顯

20、示的壓力為10.0n，取g=10m/s2（1）若上頂板傳感器的示數(shù)是下頂板示數(shù)的一半，試判斷箱的運(yùn)動(dòng)情況 （2）要使上頂板傳感器的示數(shù)為0，箱沿豎直方向的運(yùn)動(dòng)可能是怎樣的？mam圖3-5-2415如圖3-5-24所示，彈簧的勁度系數(shù)為k=600n/m，下端掛一個(gè)質(zhì)量未知的物體m，上端固定在天花板上，物體下面用質(zhì)量為m=1kg的托盤托住，托盤在外力f的作用下使彈簧恰好等于原長，然后使托盤豎直向下做加速度a大小未知的（ag）勻加速運(yùn)動(dòng)，直到托盤與物體分離已知在此過程中外力f的最小值fmin=6n，最大值為18n求：托盤與物體剛分離的瞬間物體m的速度為多大? （g取10m/s2）a圖3-5-2516

21、如圖3-5-25所示，在傾角為的光滑斜面上端系一勁度系數(shù)為k的輕彈簧，彈簧下端連有一質(zhì)量為m的小球，球被一垂直于斜面的擋板a擋住，此時(shí)彈簧沒有形變?nèi)羰殖謸醢錫以加速度a（agsin）沿斜面勻加速下滑，求：（1）從擋板開始運(yùn)動(dòng)到球與擋板分離所經(jīng)歷的時(shí)間；（2）從擋板開始運(yùn)動(dòng)到球速達(dá)到最大，球所經(jīng)過的最小路程30fba圖3-5-2617如圖3-5-26所示，質(zhì)量ma=10kg的物塊a與質(zhì)量mb=2kg的物塊b放在傾角=30的光滑斜面上處于靜止?fàn)顟B(tài)，輕質(zhì)彈簧一端與物塊b連接，另一端與固定擋板連接，彈簧的勁度系數(shù)k=400n/m，現(xiàn)給物塊a施加一個(gè)平行斜面向上的力f，使物塊a沿斜面向上做勻加速運(yùn)動(dòng)已知

22、f在前0.2s內(nèi)為變力，0.2s后為恒力，g=10m/s，求： （1）力f的最大值與最小值（2）力f由最小值達(dá)最大值過程中物塊a所上升的高度【學(xué)后反思】第1題第11題的練習(xí)目的是，了解彈性繩與剛性繩的區(qū)別，能求出瞬時(shí)作用力和瞬時(shí)加速度2第12題第14題的練習(xí)目的是，了解彈簧彈力的特點(diǎn)，求力與加速度3第15題第17的練習(xí)目的是，了解彈簧彈力的特點(diǎn)，討論在彈簧彈力的參與下物體的運(yùn)動(dòng)【參考答案】1bd 2d 3bc 4bc 5d 6cd 7bd 8略9 2mg+ 10， 11（1）0；1.5g，豎直向下，（2）f1.5mg 122.75n ，1n ，2.5m 13（1）a1=4.0m/s2，方向向右

23、（或向前）（2）a210m/s2，方向向左（或向后） 14（1）靜止或向上、向下勻速運(yùn)動(dòng)（2）m向上加速或向下減速，加速度大小大于等于10m/s2 15v=0.4m/s 16（1）（2） 17（1）fmax=100n，fmin=60n；（2）0.05mwinger 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-b

24、ack 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 in their first pool match against england on the opening day.aussie winger jarryd hayne is the competitions

25、 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 the most interest in the tournament so far.the kiwi - with a tournament high 17 offloads - has the chance of

26、 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 line this weekend, said williams.the (lack of) air up there watch mcayman islands-based webb, the head of f

27、ifas 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 match tomorrow and i have asked to meet yaya toure, he told bbc sport.for me its about how he felt and i would

28、 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, president of european footballs governing body, has also ordered an immediate investigation into the referees

29、 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 news: mission aborted.the supersonic descent could happen as early as sunda.the weather plays an important r

30、ole 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 attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather live

31、s. 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 the stratosphere. as he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.

32、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 the same mistake against the kiwis in manchester.

33、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 tournament.that could be classed as a weakness, a team under-cooked - but i have been impressed by the kangaroos f

34、ocus 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 good kick-chase. theyve been great at all the unglamorous stuff that often goes unnoticed in the stands but n

35、ot 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 full-back they have a spine that is unmatched in rugby league. they have played in so many high-pressure games

36、 - 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 oc

37、casion.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 still recovering from a knee injury aggravated against usa.both sides boast 100% records heading into the en

38、counter 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 scorer with nine, closely followed by tuivasa-sheck with eight.but it is recently named rugby league internati

39、onal 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 the first player to win the world cup in both rugby league and rugby union after triumphing with the all blac

40、ks 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-racism taskforce, is in london for the football associations 150th anniversary celebrations and will attend c

41、itys 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 speak to him first to find out what his experience was.uefa hasopened disciplinary proceedings against cskafor

42、 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.cska said they were surprised and disappointed by toures complaint. in a statement the russian side added: we

43、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 mission. starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipita

44、tion 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 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 the stratosphere. as he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.the balloon will slowly drift