理论力学中英文第一章ppt课件.ppt
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1、静力学引言Introduction to Statics,A,静力学 Statics :,静力学是研究物体的受力分析、力系的等效替换(或简化)、Statics is defined as a subject which study the force analysis, reduction of force system建立各种力系的平衡条件的科学and establish the equilibrium equations of various force system.,A,1、物体的受力分析: Force Analysis分析物体(包括物体系)受哪些力,每个力的作用位置和方向,并画出物体
2、的受力图Analyze the forces acting on the body, and identify the magnitude, direction and active point of the forces draw a Free-body diagram.,2、力系的等效替换(或简化): Reduction of force system用一个简单力系等效代替一个复杂力系。 A complex force system is replaced by a simple one.,A,3、建立各种力系的平衡条件:Establish the equilibrium equation
3、s of various force system建立各种力系的平衡条件,并应用这些条件解决静力学实际问题 Practical problems is solved by using the established equilibrium equations.,几个基本概念: Fundamental Concepts,刚体:Rigid body在力的作用下,其内部任意两点间的距离始终保持不变的物体. A rigid body is a combination of a large number of particles occupying fixed positions with respec
4、t to each other.,A,力 Force 物体间相互的机械作用,作用效果使物体的机械运动状态发生改变。A force represents the mechanical action of one body on another. The mechanical motion state of the body can be changed by it.力的内效应和力的外效应:The effect of action of the force consists of internal effect and external effect.,力的三要素:大小、方向、作用点。力是矢量Th
5、ree factors of forces are defined as magnitude, direction and point of application. Forces are vectors.,A,力系 Force system一群力可分为:平面汇交(共点)力系;平面平行力系;平面力偶系;平面任意力系;空间汇交(共点)力系;空间平行力系;空间力偶系;空间任意力系。 Force system is defined as a group of forces which act on the same body. Classification of force system Copla
6、nar force system can be classified as coplanar concurrent force system, coplanar parallel force system,coplanar force-couple system and general coplanar force system. Forces in space can be classified as spatial concurrent force system, spatial parallel force system, spatial force-couple system, gen
7、eral spatial force system.,A,平衡Equilibrium 物体相对惯性参考系(如地面)静止或作匀速直线运动。 The body is in equilibrium if it remains at rest or moves with a constant speed in a straight line relative to the inertial reference frame.,A,第一章 静力学公理和物体的受力分析 Chapter 1 Axioms of Statics and Free-body Diagram,1-1 静力学公理Section 1.1
8、 Axioms of Statics,A,公理1 力的平行四边形法则Axiom 1 The parallelogram law,作用在物体上同一点的两个力,可以合成为一个合力 Two forces acting on the same point of a body can be replaced by a resultant force合力的作用点也在该点,合力的大小和方向,由这两个力为边构成的平行四边形的对角线确定,如图所示。 . The resultant force can be obtained by constructing a parallelogram, using the t
9、wo forces as two adjacent sides of the parallelogram. The diagonal that passes through the point of application of two forces represents the magnitude and direction of the resultant force.,A,合力(合力的大小与方向) (矢量的和) Resultant force (the magnitude and direction of the resultant force) (the vector sum),亦可用
10、力三角形求得合力矢Resultant force also can be determined by force triangle rule.,A,公理2 二力平衡条件 Axiom 2 Equilibrium conditions of two forces,作用在刚体上的两个力,使刚体保持平衡的必要和充分条件是:这两个力的大小相等,方向相反,且作用在同一直线上。The sufficient and necessary equilibrium conditions of two forces acting on a rigid body arethat the two forces have
11、the same magnitude and the same line of action but opposite sense.,A,公理3 加减平衡力系原理Axiom 3 The principle of addition and subtraction of equilibrium force system,在已知力系上加上或减去任意的平衡力系,并不改变原力系对刚体的作用。The rigid body will remain unchanged if a equilibrium force system is added or subtracted from the original
12、force system acting on the rigid body.,A,推理1 力的可传性Deduction 1 The principle of transmissibility,作用于刚体上某点的力,可以沿着它的作用线移到刚体内任意一点,并不改变该力对刚体的作用。The conditions of equilibrium or of motion of a rigid body will remain unchanged if a force acting at a given point of the rigid body is replaced by a force of t
13、he same magnitude and same direction, but acting at a different point, provided that the two forces have the same line of action.,A,作用在刚体上的力是滑动矢量,The force is a sliding vector if it acts on a rigid body. 力的三要素为大小、方向和作用线The three factors of a force acting on a rigid body are magnitude, direction and
14、the line of its action.,推理2 三力平衡汇交定理Deduction 2 Principle of equilibrium of a three- force body,A,平衡时 必与 共线则三力必汇交O 点,且共面,作用于刚体上三个相互平衡的力,若其中两个力的作用线汇交于一点,则此三力必在同一平面内,且第三个力的作用线通过汇交点。 A rigid body is subjected to forces acting at only three points. It will be shown that if the body is in equilibrium and
15、 two of them are concurrent, the lines of action of the three forces must be concurrent.,A,公理4 作用和反作用定律Axiom 4 Law of action and reaction,作用力和反作用力总是同时存在,同时消失,等值、反向、共线,作用在相互作用的两个物体上The forces of action and reaction must have the same magnitude and the same line of action but opposite sense.The forces
16、 of action and reaction act on various rigid body.,在画物体受力图时要注意此公理的应用,A,公理5 刚化原理Axiom 5 Rigidified principle,变形体在某一力系作用下处于平衡,如将此变形体刚化为刚体,其平衡状态保持不变A deformable body is in equilibrium acted by a force system.The equilibrium state of the deformable body will remain unchanged if it is rigidified to a rig
17、id body.,柔性体(受拉力平衡),刚化为刚体(仍平衡),Deformable body (in equilibrium),Be rigidified ( in equilibrium),A,反之不一定成立,因对刚体平衡的充分必要条件,对变形体是必要的但非充分的,刚体(受压平衡),柔性体(受压不能平衡),Rigid body (in equilibrium),Deformable body (instability),A,1-2 约束和约束力Section 1.2 Constraints and Reactions of Constraints,约束:对非自由体的位移起限制作用的物体.,C
18、onstraint The body limits the position of the non-free body,约束力:约束对非自由体的作用力,Constraint reaction The action of the constraint to the non-free body,A,大小待定,方向与该约束所能阻碍的位移方向相反,作用点接触处,Magnitude Undetermined,Direction The direction of the constraint reaction is opposite to the direction of which the constr
19、aint can prevent the motion of the rigid body.,Point of action The contact point,约束力Constraint reaction,A,工程中常见的约束,Constraint,1、具有光滑接触面(线、点)的约束(光滑接触约束),1) Frictionless Surface,A,Constraint Reaction The direction of the force is along the normal line of the contact surface and points to the non-free
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