小班讲义高一物理第6讲平衡的应用—正交分解法

1、龙 文 教 育精品小班课程辅导讲义讲义编号 2013秋季 辅导科目： 物理 年级： 高一课 题第6讲：平衡的应用正交分解法教学目标教学重点、难点教学内容在运用正交分解法解题时，一般按如下步骤： 以力的作用点为原点作直角坐标系，标出x轴和y轴，如果这时物体处于平衡状态，则两轴的方向可根据自己需要选择，如果力不平衡而产生加速度，则x轴（或y轴）一定要和加速度的方向重合；将与坐标轴成角度的力分解成x轴和y轴方向的两个分力，并在图上标明，用符号fx和fy表示；在图上标出与x轴或与y轴的夹角，然后列出fx、fy的数学表达式。如：f与x轴夹角分别为，则。与两轴重合的力就不需要分解了；列出x轴方向上和各分力

2、的合力和y轴方向上的各分力的合力的两个方程，然后再求解。一、 运用正交分解法典型例题 例1.物体放在粗糙的水平地面上，物体重50n，受到斜向上方向与水平面成300角的力f作用，f = 50n，物体仍然静止在地面上，如图1所示，求：物体受到的摩擦力和地面的支持力分别是多少？300图1yxffgn图2 解析：对f进行分解时，首先把f按效果分解成竖直向上的分力和水平向右的分力， 对物体进行受力分析如图2所示。f的效果可以由分解的水平方向分力fx和竖直方向的分力fy来代替。则：由于物体处于静止状态时所受合力为零，则在竖直方向有：图3 则在水平方向上有： 例2.如图3所示，一物体放在倾角为的光滑斜面上，

3、求使物体下滑的力和使物体压紧斜面的力。f1g图4f2解析：使物体下滑的力和使物体压紧斜面的力都是由重力引起的，把重力分解成两个互相垂直的两个力，如图4所示，其中f1 为使物体下滑的力，f2为物体压紧斜面的力，则：例3.三个力共同作用在o点，如图6所示，f1、f2与f3之间的夹角均为600，求合力。图6f1f2f3解析：此题用正交分解法既准确又简便，以o点为原点，f1为x轴建立直角坐标；（1）分别把各个力分解到两个坐标轴上，如图7所示：f1f2xf2f3f3xf2yf3yoxy图7（2）然后分别求出 x轴和y轴上的合力（3）求出fx和fy的合力既是所求的三个力的合力如图8所示。xyffxfyo图

4、8，则合力与f1的夹角为600 第 3 次课后作业学生姓名： 30o45oabog1如图所示，用绳ao和bo吊起一个重100n的物体，两绳ao、bo与竖直方向的夹角分别为30o和40o，求绳ao和bo对物体的拉力的大小。2.（8分）如图，位于水平地面上的质量为m的小木块，在大小为f、方向与水平方向成a角的拉力作用下沿地面作匀速直线运动。求：（1） 地面对物体的支持力？（2） 木块与地面之间的动摩擦因数？3（6分）如图10所示，在倾角为=37的斜面上有一块竖直放置的档板，在档板和斜面之间放一个重力g=20n的光滑球，把球的重力沿垂直于斜面和垂直于档板的方向分解为力f1和f2，求这两个分力f1和f

5、2的大小。4.质量为m的物体在恒力f作用下，f与水平方向之间的夹角为,沿天花板向右做匀速运动，物体与顶板间动摩擦因数为，则物体受摩擦力大小为多少？8.如图所示重20n的物体在斜面上匀速下滑，斜面的倾角为370，求：（1）物体与斜面间的动摩擦因数。（2）要使物体沿斜面向上匀速运动，应沿斜面向上施加一个多大的推力？（sin370=0.6, cos370=0.8 ）10.如图所示，物体的质量，用与竖直方向成的斜向右上方的推力把该物体压在竖直墙壁上，并使它沿墙壁在竖直方向上做匀速直线运动。物体与墙壁间的动摩擦因数，取重力加速度，求推力的大小。（，）待添加的隐藏文字内容212.如图所示，物体a质量为2k

6、g，与斜面间摩擦因数为0.4若要使a在斜面上静止，物体b质量的最大值和最小值是多少？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 boa

7、st 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 scorer with nine, closely followed by tuivasa-sheck with eight.but it is re

8、cently 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 the first player to win the world cup in both rugby league and rugby union

9、 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-racism taskforce, is in london for the football associations 150th anniver

10、sary 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 speak to him first to find out what his experience was.uefa hasopened disci

11、plinary 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.cska said they were surprised and disappointed by toures complaint. in a st

12、atement 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 mission. starting at the ground, conditions have to be very calm - winds

13、 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 climb higher than the tip of mount everest (5.5 miles/8.85 kilometers), d

14、rifting 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 a

15、ssume, 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 ago they cruised through to the final and so far history has repeated its

16、elf 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 their games since then.they have been concentrating on the sort of stuff tha

17、t 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 team-mates.it is as though their entire tournament has been preparation f

18、or 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 wi

19、nning 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.winger tuivasa-sheck, who scored two tries in the kiwis 20-18 semi-final win

20、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 in their f

21、irst 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 the most in

22、terest 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 line this

23、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 match tomorro

24、w 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, president of

25、 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 news: missi

26、on 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 attached, will m

27、ove 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 stratos

28、phere. 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

29、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 tournament.that could be classed as a weakness, a t