第2課時多力平衡問題輕繩、輕桿模型[學習目標]1.熟練運用合成法、效果分解法、正交分解法處理平衡問題(重點)。2.知道輕繩、輕桿上彈力的區(qū)別，并能分析簡單的平衡問題(重難點)。一、多力平衡問題1．當物體受到不在同一條直線上的多個共點力時，一般要采用正交分解法。2．用正交分解法解決平衡問題的一般步驟：(1)明確研究對象，對物體受力分析。(2)建立坐標系：使盡可能多的力落在x、y軸上，這樣需要分解的力比較少，計算方便。(3)根據(jù)共點力平衡的條件列方程：Fx＝0，F(xiàn)y＝0。例1小王同學在家衛(wèi)生大掃除時用拖把拖地，依靠拖把對地面的摩擦力來清掃污漬。如圖所示，他沿推桿方向對拖把施加40N的推力，且推桿與水平方向的夾角θ＝37°時，剛好可以勻速推動拖把。已知拖把質(zhì)量為1kg，g取10m/s2，sin37°＝0.6，cos37°＝0.8，求：(1)拖地時地面對拖把的支持力；(2)拖把與地面間的動摩擦因數(shù)μ。________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________例2如圖所示，物體的質(zhì)量m＝4.4kg，用與豎直方向成θ＝37°的斜向右上方的推力把該物體壓在豎直墻壁上，并使它沿墻壁在豎直方向做勻速直線運動。物體與墻壁間的動摩擦因數(shù)μ＝0.5，取重力加速度g＝10m/s2，求推力F的大小。(sin37°＝0.6，cos37°＝0.8)________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________例3如圖所示，重力為G的木塊，恰好能沿傾角為θ的斜面勻速下滑，那么要將木塊沿斜面勻速向上推，必須加多大的水平推力F。________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________二、輕繩、輕桿模型如圖所示，AB、BC為輕質(zhì)桿，桿的A、C端通過鉸鏈與墻連接，兩桿都在B處由鉸鏈相連接，要使物體保持靜止，試分析AB、BC桿能否用等長的細繩代替？________________________________________________________________________________________________________________________________________________繩對物體的彈力和桿對物體的彈力有什么區(qū)別？桿的彈力方向是否一定沿著桿？________________________________________________________________________________________________________________________________________________例4(多選)一重為4N的球固定在支桿B的上端，今用一段繩子水平拉球，使桿發(fā)生彎曲，已知繩的拉力為3N，則AB桿對球的作用力()A．大小為7NB．大小為5NC．方向與水平方向成53°角斜向右下方D．方向與水平方向成53°角斜向左上方例5如圖甲所示，輕桿OB可繞B點自由轉動，另一端O點用細繩OA拉住，固定在左側墻壁上，質(zhì)量為m的重物用細繩OC懸掛在輕桿上的O點，OA與輕桿的夾角∠BOA＝30°，輕桿OB水平。圖乙中水平輕桿OB一端固定在豎直墻壁上，另一端O裝有小滑輪，用一根細繩跨過滑輪后懸掛一質(zhì)量為m的重物，圖中∠BOA＝30°，重力加速度為g，求：(1)圖甲中細繩OA的拉力和輕桿的彈力各是多大？(2)圖乙中細繩的拉力和輕桿對滑輪的作用力是多大？__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________