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1、 2013高考物理机械能错题集一、主要内容本章内容包括功、功率、动能、势能(包括重力势能和弹性势能)等基本概念,以动能定理、重力做功的特点、重力做功与重力势能变化的关系及机械能守恒定律等基本规律。其中对于功的计算、功率的理解、做功与物体能量变化关系的理解及机械能守恒定律的适用条件是本章的重点内容。二、基本方法本章中所涉及到的基本方法有:用矢量分解的方法处理恒力功的计算,这里既可以将力矢量沿平行于物体位移方向和垂直于物体位移方向进行分解,也可以将物体的位移沿平行于力的方向和垂直于力的方向进行分解,从而确定出恒力对物体的作用效果;对于重力势能这种相对物理量,可以通过巧妙的选取零势能面的方法,从而使
2、有关重力势能的计算得以简化。三、错解分析在本章知识应用的过程中,初学者常犯的错误主要表现在:“先入为主”导致解决问题的思路过于僵化,如在计算功的问题中,一些学生一看到要计算功,就只想到W= Fscos,而不能将思路打开,从W=Pt和W=Et等多条思路进行考虑;不注意物理规律的适用条件,导致乱套机械能守恒定律。例1 如图3-1,小物块位于光滑斜面上,斜面位于光滑水平地面上,在小物块沿斜面下滑的过程中,斜面对小物块的作用力A垂直于接触面,做功为零B垂直于接触面,做功不为零C不垂直于接触面,做功为零D不垂直于接触面,做功不为零【错解】斜面对小物块的作用力是支持力,应与斜面垂直,因为支持力总与接触面垂
3、直,所以支持力不做功。故A选项正确。【错解原因】斜面固定时,物体沿斜面下滑时,支持力做功为零。受此题影响,有些人不加思索选A。这反映出对力做功的本质不太理解,没有从求功的根本方法来思考,是形成错解的原因。【分析解答】根据功的定义W=Fscos为了求斜面对小物块的支持力所做的功,应找到小物块的位移。由于地面光滑,物块与斜面体构成的系统在水平方向不受外力,在水平方向系统动量守恒。初状态系统水平方向动量为零,当物块有水平向左的动量时,斜面体必有水平向右的动量。由于mM,则斜面体水平位移小于物块水平位移。根据图3-2上关系可以确定支持力与物块位移夹角大于90,则斜面对物块做负功。应选B。【评析】求解功
4、的问题一般来说有两条思路。一是可以从定义出发。二是可以用功能关系。如本题物块从斜面上滑下来时,减少的重力势能转化为物块的动能和斜面的动能,物块的机械能减少了,说明有外力对它做功。所以支持力做功。例2 以20m/s的初速度,从地面竖直向上势出一物体,它上升的最大高度是18m。如果物体在运动过程中所受阻力的大小不变,则物体在离地面多高处,物体的动能与重力势能相等。(g=10m/s2)【错解】以物体为研究对象,画出运动草图3-3,设物体上升到h高处动能与重力势能相等此过程中,重力阻力做功,据动能定量有物体上升的最大高度为H由式,解得h=9.5m【错解原因】初看似乎任何问题都没有,仔细审题,问物全体离
5、地面多高处,物体动能与重力势相等一般人首先是将问题变形为上升过程中什么位置动能与重力势能相等。而实际下落过程也有一处动能与重力势能相等。【分析解答】上升过程中的解同错解。设物体下落过程中经过距地面h处动能等于重力势能,运动草图如3-4。据动能定量解得h=8.5m【评析】在此较复杂问题中,应注意不要出现漏解。比较好的方法就是逐段分析法。例3 如图35,木块B与水平桌面间的接触是光滑的,子弹A沿水平方向射入木块后留在木块内,将弹簧压缩到最短。现将子弹、木块和弹簧合在一起作研究对象,则此系统在从子弹开始射入木块到弹簧压缩到最短的过程中A动量守恒,机械能守恒B动量不守恒,机械能不守恒C动量守恒,机械能
6、不守恒D动量不守恒,机械能守恒【错解】以子弹、木块和弹簧为研究对象。因为系统处在光滑水平桌面上,所以系统水平方向不受外力,系统水平方向动量守恒。又因系统只有弹力做功,系统机械能守恒。故A正确。【错解原因】错解原因有两个一是思维定势,一见光滑面就认为不受外力。二是规律适用条件不清。【分析解答】以子弹、弹簧、木块为研究对象,分析受力。在水平方向,弹簧被压缩是因为受到外力,所以系统水平方向动量不守恒。由于子弹射入木块过程,发生巨烈的摩擦,有摩擦力做功,系统机械能减少,也不守恒,故B正确。例4 如图36,质量为M的木块放在光滑水平面上,现有一质量为m的子弹以速度v0射入木块中。设子弹在木块中所受阻力不
7、变,大小为f,且子弹未射穿木块。若子弹射入木块的深度为D,则木块向前移动距离是多少?系统损失的机械能是多少?【错解】(1)以木块和子弹组成的系统为研究对象。系统沿水平方向不受外力,所以沿水平方向动量守恒。设子弹和木块共同速度为v。据动量守恒有mv0=(Mm)v解得v=mv0子弹射入木块过程中,摩擦力对子弹做负功 摩擦力对木块做正功 将式代入式中求得解得(2)系统损失的机械能即为子弹损失的功能【错解原因】错解中错误原因是对摩擦力对子弹做功的位移确定错误。子弹对地的位移并不是D,而D打入深度是相对位移。而求解功中的位移都要用对地位移。错解的错误是对这一物理过程中能量的转换不清楚。子弹打入木块过程中
8、,子弹动能减少并不等于系统机械能减少量。因为子弹减少的功能有一部分转移为木块的动能,有一部转化为焦耳热。【分析解答】以子弹、木块组成系统为研究对象。画出运算草图,如图37。系统水平方向不受外力,故水平方向动量守恒。据动量守恒定律有mv0=(M+m)v(设v0方向为正)子弹打入木块到与木块有相同速度过程中摩擦力做功:由运动草图可S木=S子-D 由式解得【评析】子弹和木块相互作用过程中,子弹的速度由V0减为V,同时木块的速度由0增加到V。对于这样的一个过程,因为其间的相互作用力为恒力,所以我们可以从牛顿运动定律(即f使子弹和木块产生加速度,使它们速度发生变化)、能量观点、或动量观点三条不同的思路进
9、行研究和分析。类似这样的问题都可以采用同样的思路。一般都要首先画好运动草图。例:如图3-8在光滑水平面上静止的长木板上,有一粗糙的小木块以v0沿木板滑行。情况与题中极其相似,只不过作用位置不同,但相互作用的物理过程完全一样。参考练习:如图3-9一质量为M、长为l的长方形木板B放在光滑的水平地面上,在其右端放一质量为m的小木块A,mM。现以地面为参考系,给A和B以大小相同,方向相反的初速度,使A开始向左运动,B开始向右运动,但最后A刚好没有滑离B板。求小木块A向左运动到达最远处(对地)离出发点的距离。 提示:注意分析物理过程。情景如图3-10。其中隐含条件A刚好没离B板,停在B板的左端,意为此时
10、A,B无相对运动。A,B作用力大小相等,但加速度不同,由于A的加速度大,首先减为零,然后加速达到与B同速。答案:例5 下列说法正确的是( )A合外力对质点做的功为零,则质点的动能、动量都不变B合外力对质点施的冲量不为零,则质点动量必将改变,动能也一定变C某质点受到合力不为零,其动量、动能都改变D某质点的动量、动能都改变,它所受到的合外力一定不为零。【错解】错解一:因为合外力对质点做功为零,据功能定理有EA=0,因为动能不变,所以速度V不变,由此可知动量不变。故A正确。错解二:由于合外力对质点施的冲量不为零,则质点动量必将改变,V改变,动能也就改变。故B正确。【错解原因】形成上述错解的主要原因是
11、对速度和动量的矢量性不理解。对矢量的变化也就出现理解的偏差。矢量发生变化时,可以是大小改变,也可能是大小不改变,而方向改变。这时变化量都不为零。而动能则不同,动能是标量,变化就一定是大小改变。所以Ek=0只能说明大小改变。而动量变化量不为零就有可能是大小改变,也有可能是方向改变。【分析解答】本题正确选项为D。因为合外力做功为零,据动能定理有Ek=0,动能没有变化,说明速率无变化,但不能确定速度方向是否变化,也就不能推断出动量的变化量是否为零。故A错。合外力对质点施冲量不为零,根据动量定理知动量一定变,这既可以是速度大小改变,也可能是速度方向改变。若是速度方向改变,则动能不变。故B错。同理C选项
12、中合外力不为零,即是动量发生变化,但动能不一定改变,C选项错。D选项中动量、动能改变,根据动量定量,冲量一定不为零,即合外力不为零。故D正确。【评析】对于全盘肯定或否定的判断,只要找出一反例即可判断。要证明它是正确的就要有充分的论据。例6 物体m从倾角为的固定的光滑斜面由静止开始下滑,斜面高为h,当物体滑至斜面底端,重力做功的瞬时功率为( )【错解】错解一:因为斜面是光滑斜面,物体m受重力和支持。支持不做功,只有重力做功,所以机械能守恒。设底端势能为零,则有。物体滑至底端速度为。据瞬时功率P=Fv,有P=,故选A。错解二:物体沿斜面做v0=0的匀加速运动a=mgsina。设滑到底时间为t,由于
13、,则,解得。重力功为mgh,功率为,故选B。【错解原因】错解一中错误的原因是没有注意到瞬时功率P=Fvcos。只有Fv同向时,瞬时功率才能等于Fv,而此题中重力与瞬时速度V不是同方向,所以瞬时功率应注意乘上F,v夹角的余弦值。错解二中错误主要是对瞬时功率和平均功率的概念不清楚,将平均功率当成瞬时功率。【分析解答】由于光滑斜面,物体m下滑过程中机械能守恒,滑至底端时的瞬时速度为,据瞬时功率。由图3-11可知,F、v夹角为90-。则有滑至底端瞬时功率,故C选项正确。【评析】求解功率问题首先应注意求解的是瞬时值还是平均值。如果求瞬时值应注意普遍式P=Fvcos(为F,v的夹角)当F,v有夹角时,应注
14、意从图中标明。例7 一列火车由机车牵引沿水平轨道行使,经过时间t,其速度由0增大到v。已知列车总质量为M,机车功率P保持不变,列车所受阻力f为恒力。求:这段时间内列车通过的路程。【错解】以列车为研究对象,水平方向受牵引力和阻力f。据P=FV可知牵引力F=设列车通过路程为s,据动能定理有 将代入解得【错解原因】以上错解的原因是对P=Fv的公式不理解,在P一定的情况下,随着v的变化,F是变化的。在中学阶段用功的定义式求功要求F是恒力。【分析解答】以列车为研究对象,列车水平方向受牵引力和阻力。设列车通过路程为s。据动能定理 因为列车功率一定,据可知牵引力的功率解得。【评析】发动机的输出功率P恒定时,
15、据P=FV可知v变化,F就会发生变化。牵动F,a变化。应对上述物理量随时间变化的规律有个定性的认识。下面通过图象给出定性规律。(见图3-12所示)例8 如图3-13,质量分别为m和2m的两个小球A和B,中间用轻质杆相连,在杆的中点O处有一固定转动轴,把杆置于水平位置后释放,在B球顺时针摆动到最低位置的过程中( )AB球的重力势能减少,动能增加,B球和地球组成的系统机械能守恒BA球的重力势能增加,动能也增加,A球和地球组成的系统机械能不守恒。CA球、B球和地球组成的系统机械能守恒DA球、B球和地球组成的系统机械不守恒【错解】B球下摆过程中受重力、杆的拉力作用。拉力不做功,只有重力做功,所以B球重
16、力势能减少,动能增加,机械能守恒,A正确。同样道理A球机械能守恒,B错误,因为A,B系统外力只有重力做功,系统机械能守恒。故C选项正确。【错解原因】 B球摆到最低位置过程中,重力势能减少动能确实增加,但不能由此确定机械能守恒。错解中认为杆施的力沿杆方向,这是造成错解的直接原因。杆施力的方向并不总指向沿杆的方向,本题中就是如此。杆对A,B球既有沿杆的法向力,也有与杆垂直的切向力。所以杆对A,B球施的力都做功,A球、B球的机械能都不守恒。但A+B整体机械能守恒。【分析解答】B球从水平位置下摆到最低点过程中,受重力和杆的作用力,杆的作用力方向待定。下摆过程中重力势能减少动能增加,但机械能是否守恒不确
17、定。A球在B下摆过程中,重力势能增加,动能增加,机械能增加。由于A+B系统只有重力做功,系统机械能守恒,A球机械能增加,B球机械能定减少。所以B,C选项正确。【评析】有些问题中杆施力是沿杆方向的,但不能由此定结论,只要杆施力就沿杆方向。本题中A、B球绕O点转动,杆施力有切向力,也有法向力。其中法向力不做功。如图3-14所示,杆对B球施的力对B球的做负功。杆对A球做功为正值。A球机械能增加,B球机械能减少。例9 质量为m的钢板与直立轻弹簧的上端连接,弹簧下端固定在地上。平衡时,弹簧的压缩量为x0,如图3-15所示。物块从钢板正对距离为3X0的A处自由落下,打在钢板上并立刻与钢板一起向下运动,但不
18、粘连,它们到达最低点后又向上运动。已知物体质量也为m时,它们恰能回到O点,若物块质量为2m,仍从A处自由落下,则物块与钢板回到O点时,还具有向上的速度,求物块向上运动到最高点与O点的距离。【错解】物块m从A处自由落下,则机械能守恒设钢板初位置重力势能为0,则之后物块与钢板一起以v0向下运动,然后返回O点,此时速度为0,运动过程中因为只有重力和弹簧弹力做功,故机械能守恒。2m的物块仍从A处落下到钢板初位置应有相同的速度v0,与钢板一起向下运动又返回机械能也守恒。返回到O点速度不为零,设为V则:因为m物块与2m物块在与钢板接触时,弹性势能之比2m物块与钢板一起过O点时,弹簧弹力为0,两者有相同的加
19、速度g。之后,钢板由于被弹簧牵制,则加速度大于g,两者分离,2m物块从此位置以v为初速竖直上抛上升距离 由式解得代入式解得。【错解原因】这是一道综合性很强的题。错解中由于没有考虑物块与钢板碰撞之后速度改变这一过程,而导致错误。另外在分析物块与钢板接触位置处,弹簧的弹性势能时,也有相当多的人出错,两个错误都出时,会发现无解。这样有些人就返回用两次势能相等的结果,但并未清楚相等的含义。【分析解答】物块从3x0位置自由落下,与地球构成的系统机械能守恒。则有v0为物块与钢板碰撞时的的速度。因为碰撞板短,内力远大于外力,钢板与物块间动量守恒。设v1为两者碰撞后共同速mv0=2mv1 (2)两者以vl向下
20、运动恰返回O点,说明此位置速度为零。运动过程中机械能守恒。设接触位置弹性势能为Ep,则同理2m物块与m物块有相同的物理过程碰撞中动量守恒2mv0=3mv2 (4)所不同2m与钢板碰撞返回O点速度不为零,设为v则因为两次碰撞时间极短,弹性形变未发生变化Ep=Ep (6)由于2m物块与钢板过O点时弹力为零。两者加速度相同为g,之后钢板被弹簧牵制,则其加速度大于g,所以与物块分离,物块以v竖直上抛。据运动学公式,则由得 (7)【评析】本题考查了机械能守恒、动量守恒、能量转化的。守恒等多个知识点。是一个多运动过程的问题。关键问题是分清楚每一个过程。建立过程的物理模型,找到相应解决问题的规律。弹簧类问题
21、,画好位置草图至关重要。参考练习:如图3-16所示劲度系数为k1的轻质弹簧分别与质量为m1,m2的物体1,2,栓接系数为k2的轻弹簧上端与物体2栓接,下端压在桌面上(不栓接)。整个系统处于平衡状态,现施力将物体1缓慢地竖直上提,直到下面那个弹簧的下端刚脱离桌面,在此过程中,物体2的重力势能增大了多少?物体1的重力势能增大了多少?提示:此题隐含的条件很多,挖掘隐含条件是解题的前提。但之后,必须有位置变化的情景图如图3-17。才能确定1,2上升的距离,请读者自行解答。空喊钓鱼岛是中国的,当然是没用的,我们当然可以用武力收复,我们有信心,得最终看有没有这个能力?我们期待着!空喊股市雄起是没用的,悲惨
22、的是股民除了空喊之外,5年来,赔的连后悔的机会都没有!但愿十八大是股市转折的契机!期待2013年高考英语快速提分而没有立竿见影的系统秘术也是没用的,除了被汉语广告文字诱惑外,从众,冲动,买到的只是带光盘的名师讲座而已(与重点高中在职教师讲大课知识点串讲无异)!莫言,获得了诺贝尔文学奖,成功了!莫急,吴军高考英语秘术,5招满分搞定7选5,是否觉着不可思议?答案总是出乎意料的!正如:问题:一个美丽的女子大热天上身穿一件羽绒服,下身穿的却是薄丝袜,为什么?一个合理而又搞笑的回答:因为鲜奶要加热,火腿要冷藏!永无止境的创新与挑战! 总是被模仿,从未被超越!2013高考英语7选5阅读填空题满分秘术 答题
23、步骤:1)快速阅读全文要点词句,包括首尾段、首尾句以及独立成段的句子和文章中带转折词的句子等,目的是掌握文章的主旨。2)然后再边读边做题,重点阅读各个问题附近的句子,圈定线索词,然后从选项中寻找相关的特征词,以确定答案。做题时可以采用代入排除法。如果一题做不出或拿不准,可先放过,继续往下读,先做容易的能做出的题,直到读完整篇文章。至此,文章的要点和主旨、各个段落之间的逻辑关系应基本清楚了。3) 4) 5)答题技巧: 秒杀秘术: 效果超出您的相像!最快5秒选中答案!马上拿2012全国各地7选5真题验证吧!How to Make Friends 【2012辽宁卷】Friendship is a v
24、ery important human relationship and everyone needs good friends. Good friendship has many benefits. It offers companionship, improves self-worth and promotes good health. There are times in our lives such as when we have recently moved into a new town, or changed our jobs or schools. Such changes o
25、ften leaves us without a friend 71 . But for many of us the process is difficult and requires courage. Below are some helpful suggestions on how to make and keep friends. Associate with others. The first step to making friends is associating with other people. You can go to public places to meet new
26、 people. Besides, you will need to make yourself known by becoming an active member of such places. Start a conversationStarting a conversation is the second most important step in making new friends. 72 You can always start the conversation. Being able to make small talk is a very useful skill in r
27、elating with other people. 73 Choosing friends with common interests is important in building friendship as these interests would always bring you and your friend together, Hanging out will always be a pleasant experience. Let it grow. It is a good thing to stay in touch. However, try not to press y
28、our new friend with calls, messages or visits as this would likely wear him or her out and finally you may lose your friend. 74 . The best friendships are the ones that grow naturally. Enjoy your friendshipThe best way to enjoy your friendship is to allow your friends to be themselves. 75 Try not to
29、 change them from who they are to what you want them to be. Become the kind of friend you will want your friend to be to you. A. Be cheerful. B. Do things together. C. Do not wait to be spoken to. D. Try not to find fault with your friends. E. Making new friends comes easy for some people. F. For a
30、friendship to develop you need to stay in touch. G. So you will need to give your friend time to react to you. 73空: 段落间结构排比 + 复现Start a conversationStarting a conversation is the second most important step in making new friends. 72 You can always start the conversation. Being able to make small talk
31、 is a very useful skill in relating with other people. 73 Choosing friends with common interests is important in building friendship as these interests would always bring you and your friend together, Hanging out will always be a pleasant experience. Let it grow. It is a good thing to stay in touch.
32、 However, try not to press your new friend with calls, messages or visits as this would likely wear him or her out and finally you may lose your friend. 74 . The best friendships are the ones that grow naturally. A. Be cheerful. B. Do things together. C. Do not wait to be spoken to. D. Try not to fi
33、nd fault with your friends. E. Making new friends comes easy for some people. F. For a friendship to develop you need to stay in touch. G. So you will need to give your friend time to react to you. 72空: 同现,与段中心沾边!Start a conversationStarting a conversation is the second most important step in making
34、 new friends. 72 You can always start the conversation. Being able to make small talk is a very useful skill in relating with other people. A. Be cheerful. B. Do things together. C. Do not wait to be spoken to. D. Try not to find fault with your friends. E. Making new friends comes easy for some peo
35、ple. F. For a friendship to develop you need to stay in touch. G. So you will need to give your friend time to react to you. 74空: 指代 + 复现!Let it grow. It is a good thing to stay in touch. However, try not to press your new friend with calls, messages or visits as this would likely wear him or her ou
36、t and finally you may lose your friend. 74 . The best friendships are the ones that grow naturally. A. Be cheerful. B. Do things together. C. Do not wait to be spoken to. D. Try not to find fault with your friends. E. Making new friends comes easy for some people. F. For a friendship to develop you
37、need to stay in touch. G. So you will need to give your friend time to react to you. 75空: 排笔 + 指代 + 复现!Enjoy your friendshipThe best way to enjoy your friendship is to allow your friends to be themselves. 75 Try not to change them from who they are to what you want them to be. Become the kind of fri
38、end you will want your friend to be to you. A. Be cheerful. D. Try not to find fault with your friends. E. Making new friends comes easy for some people. F. For a friendship to develop you need to stay in touch. G. So you will need to give your friend time to react to you. 71空: 转折关系定位!【2012辽宁卷】Frien
39、dship is a very important human relationship and everyone needs good friends. Good friendship has many benefits. It offers companionship, improves self-worth and promotes good health. There are times in our lives such as when we have recently moved into a new town, or changed our jobs or schools. Su
40、ch changes often leaves us without a friend 71 . But for many of us the process is difficult and requires courage. Below are some helpful suggestions on how to make and keep friends. A. Be cheerful. E. Making new friends comes easy for some people. G. So you will need to give your friend time to rea
41、ct to you. Empathy【2012北京卷】Last year, researchers from the University of Michigan reported that empathy, the ability to understand other people, among college students had dropped sharply over the past 10 years. _71_ Today, people spend more time alone and are less likely to join groups and clubs.Je
42、nnifer Freed, a co-director of a teen program, has another explanation. Turn on the TV, and youre showered with news and reality shows full of people fighting, competing, and generally treating one another with no respect. _72_There are good reasons not to follow those bad examples. Humans are socia
43、lly related by nature. _73_ Researchers have also found that empathetic teenagers are more likely to have high self-respect. Besides, empathy can be a cure for loneliness, sadness, anxiety, and fear.Empathy is also an indication of a good leader. In fact, Freed says, many top companies report that e
44、mpathy is one of the most important things they look for in new managers. _74_ “Academics are important. But if you dont have emotional (情感的) intelligence, you wont be as successful in work or in your love life,” she says.Whats the best way to up your EQ (情商)? For starters, let down your guard and r
45、eally listen to others. _75_To really develop empathy, youd better volunteer at a nursing home or a hospital, join a club or a team that has a diverse membership, have a “sharing circle” with your family, or spend time caring for pets at an animal shelter.A.Everyone is different, and levels of empathy differ from person to person.B.That could be because so many people have replaced face time with screen time, the researchers said.C.“One doesnt develop empathy by having a lot of opinions and doing a lot of talking,” Freed says.D.Humans learn by exampl
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