1、华中师范大学物理科学与技术学院08级毕业生秦伟,*,华中师范大学物理科学与技术学院08级毕业生秦伟,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,第三章 相互作用,第2节:弹力,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,力的作用效果,第三章 相互作用,复习:,使物体的形状发生变化,改变物体的运动状态,Evaluation only.,Created with A
2、spose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,第三章 相互作用:弹力,一、形变,新课,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,二、力的作用效果:可以使物体发生形变,形变的举例,1、被拉长或压缩的弹簧,2、弯曲的木棍,3、压缩的气球,4、压缩的橡皮泥,第三章 相互作用,Evaluati
3、on only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,定义:由于外力作用下,物体形状、体积大小的变化称为形变。,一切物体,在外力作用下都会发生形变?,形变:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,明显形变,微小形变,形变分类一:,明显形变容易看
4、到,微小形变难以,直接,观察,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,微小形变的观察,微小形变难以直接观察,但可以,间接,引起一些明显的现象,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,微小形变的,实验演示,:,1、课本
5、演示实验,2、玻璃杯被压缩后变形,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,有一些物体眼睛根本观察不到它的形变,比如一些比较坚硬的物体,但是这些物体都有形变,只不过形变很微小,。
6、,一切物体受到外力作用都要发生形变,微小形变的实验演示:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,弹性形变:,能恢复到原来形状的形变,非弹性形变:,不能恢复到原来形状的形变,非弹性形变,1,2,形变分类二:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011
7、Aspose Pty Ltd.,一切物体,发生,弹性,形变都会产生力的作用,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,二、弹力,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(1)定义:,发生弹性形变的物体由于要恢复原来的形
8、状,对与它接触的物体发生力的作用,这种力叫做弹力。,强调:,弹力不是指“弹簧产生的力”。,某一弹力的产生是,施力物发生弹性形变,产生的。,弹力:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,例如:,F2,F,1,图中球所受弹力是谁发生形变产生的?,圆球,墙,斜面,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2
9、.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(2)产生条件:,直接接触,发生,弹性,形变,弹力:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,如何判断有无弹力?,弹力:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 A
10、spose Pty Ltd.,.,例1:,(1),条件:,直接接触 发生弹性形变,是否发生了挤压形变?,可采用假设法,假设没有这面墙,球动了么?,能否保持原状?,F,只有支持力,F,弹力:,假设没有下,面的墙呢,.,F,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(2),例2:,条件:,直接接触 发生弹性形变,采用假设法:,F,1,把墙拆了。,F2,弹力:,说明受到了墙的弹力,如果抽掉斜面呢?,说明受到了斜面
11、的弹力,F,1,球受到墙和斜面两个弹力的作用,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,说明只受绳子的拉力,0,例3:,条件:,直接接触 发生弹性形变,弹力:,假设去掉斜面,0点有弹力么?,球保持原状,说明0点,假设绳子断开,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 20
12、04-2011 Aspose Pty Ltd.,(3)弹力的种类:,按效果分为,压力,支持力,拉力,弹力:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,三、弹力的三要素,大小,作用点,方向,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty
13、Ltd.,弹力的大小与什么有关?,弹性形变越大,弹力越大?,1、弹力的大小:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,弹性限度:,如果形变过大,超过一定的限度,撤去外力后,物体就不能完全恢复原来的形状,这个限度叫做弹性限度。,1、弹簧弹力的大小:,不超过弹性限度时,弹性形变越大,弹力越大!,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Cl
14、ient Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,弹簧,的弹力大小,胡克定律,:,F=KX,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,1、K称为劲度系数,单位N/m,由弹簧的材料,粗细,长度等自身性质决定。,2、既适用于弹簧拉伸,也适用于弹簧压缩,X仅仅是形变的改变量。,弹簧的弹力:,F=K,X,弹簧,的弹力大小,胡克定律,:,Evaluat
15、ion only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,练习:,3cm,3.2cm,50N,注意:,弹簧的弹力满足F=KX,弹簧伸长或压缩对两端物体作用力大小相等。,弹簧伸长不可超过最大伸长量。,1、弹力的大小:,一根弹簧原长3cm,挂一重为50N的勾码伸长到3.2 cm,求挂4个勾码伸长了多少?,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profil
16、e 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,下列情况的弹力大小:,1、竖直悬挂,质量为2Kg的物体静止时拉力大小。,1、弹力的大小:,M=2Kg,原理:二力平衡,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,注意:,虽然以上例题中,,拉力、支持力、压力,大小等于重力,但并,不是重力,,而是弹力!,1、弹力的大小:,Evaluation only.,Created
17、 with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,两物体间的弹力是发生在相互接触面上,但可以将作用点移到重心上。,2、弹力的作用点:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,.,(1),F,2、弹力的作用点:,(2),F,1,F,2,.,可以将作用点移到重心上,Evalua
18、tion only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,2、弹力的作用点:,F,拉力,.,可以将作用点移到重心上,(3),Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,弹力的方向:,弹力是由于物体发生弹性形变,趋于恢复原状而产生的,因而弹力方向指向形
19、变趋于恢复的方向。,弹力方向,形变趋于恢复的方向,3、弹力的方向:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,几种常见的弹力,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,1.拉力,物体受到的拉力:,由,什么物体形变产生的?
20、,可见拉力也是弹力,方向:绳子对物体的拉力是沿着绳而指向绳收缩的方向。,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,2.压力和支持力,桌面受到的的压力:,由,什么物体形变产生的?,物体受到的支持力:,由,什么物体形变产生的?,可见压力和支持力都是弹力,方向:垂直于接触面指向被压或被支持的物体,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Cli
21、ent Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(3)各种,接触面,间的弹力方向判断,3、弹力的方向:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,平面,与,平面,间弹力方向:,(3)各种接触面间的弹力方向:,垂直平面指向受力物体,摩擦力,Evaluation only.,Created with Aspose.Slides for.NET 3
22、.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,曲面,与,平面,间弹力方向:,(3)各种接触面间的弹力方向:,.,例(1),F,n,过接触点垂直平面指向受力物体,物体所受的弹力:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,与过接触点的公切面垂直,并指向受力物体,N,A,B,N,N,B对A,(3)各种接触面间的弹力方向:,曲面,与,
23、曲面,间弹力方向:,半球形的碗,分析A球受力,做切线,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,N,N,过接触点垂直平面指向受力物体,光滑斜面,A,B,N,A,N,B,(3)各种接触面间的弹力方向:,点,与,平面,间弹力方向:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2
24、004-2011 Aspose Pty Ltd.,动动脑、想一想:,弹簧,类物体的弹力方向,绳类,物体的弹力方向,木棍、铁棒、,杆,的弹力方向,各种,接触面,之间的弹力方向,不同材质的物体,弹力方向各不相同,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,弹簧与绳子弹力的异同点:,弹簧或绳子的拉力方向,与弹簧或绳子的收缩方向相同。,相同点:,弹簧可以被压缩,绳不能。,形变不同,弹簧的弹力不会立即消失,绳子属于微小
25、形变,恢复快,可以立即消失。,不同点:,动动脑、想一想:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(1),弹簧,与,绳,子,A,A,B,B,轻绳,只能产生拉力,方向沿绳指向绳收缩的方向,形变属于,微小形变,,,绳断瞬间,力也瞬间消失,。,弹簧,产生的压力或拉力方向沿弹簧的轴线,形变属于,明显形变,,绳断瞬间,力不会马上消失。,动动脑、想一想:,绳断瞬间,Evaluation only.,Created w
26、ith Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,(2)杆的作用力与绳子弹力的区别,A,轻杆既可压缩,或弯曲产生支持力,又可伸长产生拉力,方向与形变方向相反。,绳子只能伸长产生拉力!,杆子的弹力不一定沿杆。,垂直杆向上,B,C,杆的弹力方向如何根据运动状态决定?,动动脑、想一想:,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004
27、-2011 Aspose Pty Ltd.,1、下列关于弹力的说法,不,正确是,?,A、只要两个物体接触就一定能产生弹力,B、两个接触并发生弹性形变的物体一定产生弹力,C、压力、支持力、绳的拉力都是弹力,D、压力、支持力的方向总是垂直于支持面,A,例与练,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,完成下列物体受弹力的示意图:,例与练,Evaluation only.,Created with Aspose.
28、Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,.,完成下列物体受弹力的示意图:,例与练,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,例与练,A,B,固学案T8:如图所示,绳子、滑轮质量和摩擦不计,物体A重4N,物体B重1N,下列说法正确的是(),地面对A的支持力是3N。,测力计示数为2N,物体A底部
29、形变向下,所以对地压力向下。,物体A受拉力为4N。,4N,1N,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,小结,一、弹力产生条件:,直接接触,发生弹性形变,二、弹力方向,1、压力和支持力:,方向都垂直于接触面指向被压或被支持的物体。,2、拉力:,绳的拉力沿着绳指向绳 收缩的方向,Evaluation only.,Created with Aspose.Slides for.NET 3.5 Client Profile 5.2.0.0.,Copyright 2004-2011 Aspose Pty Ltd.,