§5巧用定性分析法解物理问题

§5巧用定性分析法解物理问题 ?5巧用定性分析法解物理问题 作为分析思维的一种形式，定性分析就是在一定的条件下，根据概念规律作逻辑推理，这是学好物理，，能做到悟理、讲理的基础，也是一种解决物理问题的一种思维方法。纵观近年来的高考，运用定性判断的方法求解的题目逐年增多，因此加强定性分析能力的培养和训练，是物理教学中值得十分重视的问题。 一、 重点难点的透析 1、本专题涉及的范围广、多，几乎整个高中物理的各个部分均涉及到，但概括起来，大致分为5类: 第一类:是有关物理现象的定性分析题。物理现象是物理过程的“直接记录”，虽然它还没有揭示现象的规律，但是它总是按照一定的规律发展变化着，因此解答这类定性分析题的重要方法是，要善于透过现象抓住实质，用物理概念和规律说明物理现象为什么会发生和怎样发生。 第二类:是用于巩固、加深对物理概念、规律的理解的定必分析题。这类题目的特点是给出多种说法，其中有正确的说法，错误的说法，似是而非，使答题者在选择时犹豫不定，受到迷惑而出现错误。命题者为了考查学生掌握知识的程度或发现学生在知识上的漏洞，设下种种“知识陷井”。解答这类题目时，首先要深刻理解物理概念和规律的“内涵”和“外延”，即弄清概念的来龙去脉。第二，要善于识别真伪，因这类题为了达到巩固概念和规律的目的，常常给出许多貌似正确、合理的“理由”、“推论”，读者必须从貌 似正确之中找出其不正确之处。第三:举例反驳是一种常用的分析方法，对有些绝对肯定或绝对否定的定性题的结论，答题时如能举出一个否定的例子，往往很快就能把问题说清。第四:掌握一定的解题技巧，使这类题能快速作答。 第三类:是用于说明物理实验、物理仪器和物理装置的原理、分析实验结果的定性分析题。 第四类:一些定量分析的题目，必须先用定性分析的方法分析好题意，弄清物理过程，然后才能确立定量关系。列方程求解，可风定性分析是求解这类题的第一步。 第五类:有不少带数据的计算题，乍一看似乎要通过定量计算才能作答，其实拟题者的良苦作心，正是要考查学生定性分析的判断的能力。 2、应注意的问题: 不管哪一类定性分析题，答案都 必须注意科学性、条理性、规范性。 (1)科学性:就是要求答案尽量准确地运动物理概念和规律，防止用生活语言代替物理原理，防止就事论事，而对物理原理却不沾边。 (2)条理性:就是答案要逻辑合理，层次分明，论点突出，论据充分，防止思维混乱，主次不分，语无伦次。 (3)规范性:就是答案要准确应用字、词、句、标点、言简意赅、书写规范，防止文句不通，词不达意。不写错别字。 office, branch offices (jurisdiction), risk management, marketing management sector through supervision and inspection found problems, should be assigned the investigators are corrected in a timely manner. 27th the fifth chapter penalty under any of the following acts, then the relevant provisions to punish the investigators, according to the Bank. To constitute a crime shall be investigated for criminal responsibility: (A) on the business that are not involved in the investigation, issued a survey. (B) customer credit information are not thorough verification. (Iii) to participate in credit customer survey is not in place, customers and data is incomplete, untrue; he knows bear a counterfeited clients issuing credit. (D) does not provide for due diligence of credit business, pre-loan investigation form, concealing facts or providing false information or should be found in a normal investigation failed to discover the risk factors, lead to the review and approval policy errors, loan risk. (Five) on loan guarantees of survey not in place, not by provides on arrived, and pledge real for field verification, and assessment, and identification and registration, not according to provides on guarantor of guarantees qualification and guarantees capacity for survey verified, led to guarantees loan lost authenticity, and legitimacy, and effectiveness of; cycle loan business in the of mortgage 二、 解题方法和技巧 1、 解题的一般思路 (1)分析研究对象，确定解题目标。 物理问题总是有“物”有“理”，“物”、“理”渗透。我们分析物理问题，就是要 揭示“物”中之“理”，明察 “理”中之“物”，因此在分析题意的基础上，选定研究 对象是第一重要步骤。 (2)分析临界状态，抓住某个承前启后的物理量，这样也就抓住了解题关键。 (3)分析物理过程，理顺解题思路。 物理现象的发生、变化的过程，叫物理过程。分析物理过程主要抓三性:?阶段性——弄清一个物理过程可分为哪 几个阶段;?联系性——找出各个阶段之间是由什么物理量联系起来的;?规律性——明确每个阶段遵循什么规律。 (4)根据物理过程的特征，选用相关规律求解。 (5)检验答案是否合理。 2、 几种重要方法和技巧。 (1) 定性比较，快速作答。 有些物理习题，题目本身并不要求出物理量的确切值，往往侧重于快慢、大小、高低的比较，在解答这类习题时，运用分析法作答，可简化物理过程，提高解题的速度。 (2)逻辑推理顺藤摸瓜 从物理过程变化的初始条件出发，通过逻辑推理，明晰物理过程的变化规律，步步为营，环环相扣。由因索果，顺藤摸瓜。 (3)特殊定性，一目了然。 对特殊情形的讨论，是定性分析 的一种特殊方法，特殊 定性，能剥去题目的外衣，迅速把握问题的本质，尤其在解用字母达的单选题时，运用特殊的定性，可使问题的答案一目了然。 (4)运用假想，实现突破 假想是人们以已有的和物理理论为依据，对未知事物的规律性所作的假定性解释。假想方法是定性判断的一种有效的方法，巧用假想法进行定性分析，可突破问题的难点，简化解题过程，提高解题效率。 (5)定性作图，解答难题。 有些物理习题，根据题给条件，试图列式推演，找不到列式的突破口或利用初等列不出方程，这样，解题就会陷入“山穷水尽”的困境，这时，如果我们转化思维定性，定性作出描述物理规律的图像或图解，进而考虑图像的意义，就会“柳暗花明”、“烟消云散”，使 问题得到 顺利解决。 (6)整体分析，迎刃而解。 有些物理问题，如用隔离法分析，由于涉及物体间的相互作用力，需要经过繁琐的数学推导，增加判断难度，若采用整体分析，问题将迎刃而解。 acy, and effectiveness of; cycle loan business in the of mortgagef guarantees qualification and guarantees capacity for survey verified, led to guarantees loan lost authenticity, and legitimntor od pledge real for field verification, and assessment, and identification and registration, not according to provides on guaraeview and approval policy errors, loan risk. (Five) on loan guarantees of survey not in place, not by provides on arrived, anto discover the risk factors, lead to the rloan investigation form, concealing facts or providing false information or should be found in a normal investigation failed -clients issuing credit. (D) does not provide for due diligence of credit business, pre feited) to participate in credit customer survey is not in place, customers and data is incomplete, untrue; he knows bear a countert are not involved in the investigation, issued a survey. (B) customer credit information are not thorough verification. (Iiiss thaestigators, according to the Bank. To constitute a crime shall be investigated for criminal responsibility: (A) on the businetimely manner. 27th the fifth chapter penalty under any of the following acts, then the relevant provisions to punish the invproblems, should be assigned the investigators are corrected in a office, branch offices (jurisdiction), risk management, marketing management sector through supervision and inspection found 2 (7)“等效”分析，变难为易。 等效法是物理分析中应用最广泛的方法之一。这种方法是具体问题中应用 的关键是:确定等效的物理量，找出具有这种等效性质的变换形式。 三、典型例题分析 【例1】一个以速度V竖直上抛的物体，在上升和下降过程中的阻力大小相等，那么0 物体上升的时间t和下落的时间t之间的关系正确的是: 12 A)t< t B) t= t C) t> t D)条件不足，无法确定。 121212 【解析】由于有阻力，物体抛出后落回抛出点的速度V必然是Vt,即:乙球先到达B。应选B。 12 【说明】本例中的两球均为变速曲线运动，运动情况较为复杂，难以运用公式通过计算运动时间来比较到B的先后，因此解本题的关键是将给出的已知条件转化为图像，再利 egitimacy, and effectiveness of; cycle loan business in the of mortgageand lguarantor of guarantees qualification and guarantees capacity for survey verified, led to guarantees loan lost authenticity, ed, and pledge real for field verification, and assessment, and identification and registration, not according to provides onarriv ead to the review and approval policy errors, loan risk. (Five) on loan guarantees of survey not in place, not by provides onling facts or providing false information or should be found in a normal investigation failed to discover the risk factors, lloan investigation form, concea-ounterfeited clients issuing credit. (D) does not provide for due diligence of credit business, pre. (Iii) to participate in credit customer survey is not in place, customers and data is incomplete, untrue; he knows bear a ccationbusiness that are not involved in the investigation, issued a survey. (B) customer credit information are not thorough verifihe investigators, according to the Bank. To constitute a crime shall be investigated for criminal responsibility: (A) on the nish trected in a timely manner. 27th the fifth chapter penalty under any of the following acts, then the relevant provisions to pu, marketing management sector through supervision and inspection found problems, should be assigned the investigators are coroffice, branch offices (jurisdiction), risk management3 用图像推理比较，问题就变得易解了。 【例4】如图，电源电动势为ε内阻为r，当滑线变阻器 V R向左移动时，图中各表读数变化正确的是: 3A1 A) 表A、V变大，表A、v、V变小; 1122 B) 表A、A、V变大，表V、V变小; 1212RR2 1 C) 表A变大，表A、V、V、V变小; 1212A2 R3 D) 表A、A、V、V变大，表V变小。 1221V1 V2 【分析】由图知，当滑线变阻器滑动端向左移动时，R的3 值变小，总电阻变小，电源的路端电压变小，即表V的读数变小;表A的读数变大，R11的阻值不变，则表V的读数变大，表V的读数变小，R的电流减小，表A的读数是增大的，1221所以表A读数增大。所以先选B。 2 【说明】运用欧姆定律定性分析电路中各部分的电流、电压的变化情况，可按以下思路进行:(1)电路中不论是串联还是并联，只要有一个电阻变小，那么总电阻将变小;(2)由全电路欧姆定律可判断总电流、总电压是变大还是变小;(3)判断固定的分流电路中的电流、电压变化;(4)判断变化部分的电流、电压，如变化部分是并联电路，那么仍旧先判断固定电阻部分的变化，自然电阻变化的部分的电流、电压就可以知道。 四、针对训练 1、物体以60J的初动能从地面竖直上抛，设空气阻力恒定，它上升到某一高度时，动能损失50J，而机械能损失了10J，则物体落回地面时的动能可能是: A)50J B)48J C)40J D)36J。 2、如图5 所示，在粗糙的水平面上有一三角形木块abc，在它们的两 个粗糙的斜面上分别放两个质量为m、m的木块，m> m，已知三角形木块和两 物体都静止，则粗糙平1212 面对三角形木块: A) 有摩擦力作用，摩擦力方向水平向左; a B) 有摩擦力作用，摩擦力方向水平向右; m1m 2A) 有摩擦力作用，但摩擦力方向不能确定因m、1 θ 2θ 1m、θ、数值未给出; 21c b B) 以上结论都不对。 3、如图，R=75Ω,R=125Ω，变阻器R的总电阻为150Ω，安培表的内电阻不计，当12 变阻器的滑动片P从最右端移到最左端时，安培表的示数范围是: A)0-0.21A R1 B)0-0.05A P C)0.05A-0.21A A R R2 D)0.21A-0.05A 10V 4、如图，在光滑的水平面上，质量为2Kg的小车以8m/s的速度向右运动，与静止的 acy, and effectiveness of; cycle loan business in the of mortgagef guarantees qualification and guarantees capacity for survey verified, led to guarantees loan lost authenticity, and legitimntor od pledge real for field verification, and assessment, and identification and registration, not according to provides on guaraeview and approval policy errors, loan risk. (Five) on loan guarantees of survey not in place, not by provides on arrived, anto discover the risk factors, lead to the rloan investigation form, concealing facts or providing false information or should be found in a normal investigation failed -clients issuing credit. (D) does not provide for due diligence of credit business, pre feited) to participate in credit customer survey is not in place, customers and data is incomplete, untrue; he knows bear a countert are not involved in the investigation, issued a survey. (B) customer credit information are not thorough verification. (Iiiss thaestigators, according to the Bank. To constitute a crime shall be investigated for criminal responsibility: (A) on the businetimely manner. 27th the fifth chapter penalty under any of the following acts, then the relevant provisions to punish the invproblems, should be assigned the investigators are corrected in a office, branch offices (jurisdiction), risk management, marketing management sector through supervision and inspection found 4 质量为5Kg的小车B发生正碰，若碰撞过程中，系统坳能损失最大，取向右的方向为正方向，则A和B的速度分别是: //A) V=0，V= 4m/s ABB A //B) V=0，V=0 AB//C) V=-2m/s，V=6m/s AB//D) V= V=3m/s AB 5、如图，托盘秤上放一个密闭容器，其内装满水，用细绳系一只乒乓球固定在底部，乒乓球悬浮在水中，这时托盘秤有一确切的读数，若细绳突然断了，在乒乓球上浮的过程中，托盘秤的读数将: A) 不变; B) 增大; C) 减小; D) 先减小后增大; E) 先增大后减小，最后恢复原读数。 6、一个初动能为E的带电粒子，以速度V垂进电场线飞入平行板电容器，飞出电容K00 器时的动能是初动能的2 倍，如果此带电粒子的初速度增至原来的2 倍，那么当它飞出电容器时的动能变为: 10E B)8 E C)5 E D)25 A)K0K0K0B A E。 K0 7、在平行水平地面的有界匀强磁场上方，有两个用× × × × × × 相同金属材料制成的边长相同的正方形线圈A和B，不同× × × × × × 之处是线圈A的导线比线圈B的导线粗，当两个线圈从相× × × × × × 同的高度由静止开始下落，如图，试比较它们落地的时间 的大小。 8、如图，某物体从一高度处由静止开始滑下，第一次A 经过光的斜面A、B，滑下底端的时间为t第二次经过光滑1， 曲面ACD滑下，滑至底端需时间t,设两次通过的路程相等，2C 试比较t、t的大小。 12B O D egitimacy, and effectiveness of; cycle loan business in the of mortgageand lguarantor of guarantees qualification and guarantees capacity for survey verified, led to guarantees loan lost authenticity, ed, and pledge real for field verification, and assessment, and identification and registration, not according to provides onarriv ead to the review and approval policy errors, loan risk. (Five) on loan guarantees of survey not in place, not by provides onling facts or providing false information or should be found in a normal investigation failed to discover the risk factors, lloan investigation form, concea-ounterfeited clients issuing credit. (D) does not provide for due diligence of credit business, pre. (Iii) to participate in credit customer survey is not in place, customers and data is incomplete, untrue; he knows bear a ccationbusiness that are not involved in the investigation, issued a survey. (B) customer credit information are not thorough verifihe investigators, according to the Bank. To constitute a crime shall be investigated for criminal responsibility: (A) on the nish trected in a timely manner. 27th the fifth chapter penalty under any of the following acts, then the relevant provisions to pu, marketing management sector through supervision and inspection found problems, should be assigned the investigators are coroffice, branch offices (jurisdiction), risk management5