学 说明:①对刚体来说,上面的条件是充要的 ②对变形体来说,上面的条件只是必要条件(或多体中) F F (变形体)绳 多体 ③二力体:只在两个力作用下平衡的刚体叫二力体。 F F 力体 F F1二力杆 16
16 说明：①对刚体来说，上面的条件是充要的 ③二力体：只在两个力作用下平衡的刚体叫二力体。 ②对变形体来说，上面的条件只是必要条件(或多体中) 二力杆
Statics 2nd Principle: addition and subtraction of balanced force systems The action of a given force system on a rigid body remains unchanged if another balanced force system is added to or subtracted from the original system Corollary l: transmissibility of a force The point of application of a force acting on a rigid body can be transferred to any other point on the line of action of the force without altering its effect B B F Fi/F2 F2=F=-F1 Therefore, as far as a rigid body is concerned a force can be characterized by three elements magnitude, direction and line of application 7
17 The action of a given force system on a rigid body remains unchanged if another balanced force system is added to or subtracted from the original system. Corollary 1: transmissibility of a force The point of application of a force acting on a rigid body can be transferred to any other point on the line of action of the force without altering its effect. Therefore, as far as a rigid body is concerned, a force can be characterized by three elements: magnitude, direction and line of application. 2nd Principle: addition and subtraction of balanced force systems
学 公理2加减平衡力系原理 在已知力系上加上或减去任意一个平衡力系,并不改变原 力系对刚体的作用。 推论1:力的可传性。 作用于刚体上的力可沿其作用线移到同一刚体内的任一 点,而不改变该力对刚体的效应。 B B B F F Fi/F2 F2=F=-F1 因此,对刚体来说,力作用三要素为:大小,方向,作用线 18
18 在已知力系上加上或减去任意一个平衡力系，并不改变原 力系对刚体的作用。 推论1：力的可传性。 作用于刚体上的力可沿其作用线移到同一刚体内的任一 点，而不改变该力对刚体的效应。 因此，对刚体来说，力作用三要素为：大小，方向，作用线 公理2 加减平衡力系原理
Statics 3rd principle: the law of parallelogram Two forces applied at one point of a body have as their resultant a force applied at the R same point and represented by the diagonal of a parallelogram constructed with two F2 given forces as Its sides r=E+F R Corollary 2: three equivalent forces intersect A at one point If a free rigid body remains in equilibrium under the action of three nonparallel coplanar Fi forces the lines of action of those forces F3 intersect at one point. Moreover, three forces ar coplanar. (under special circumstances, forces intersect at infinite and become a parallel force system) 19
19 If a free rigid body remains in equilibrium under the action of three nonparallel coplanar forces, the lines of action of those forces intersect at one point. Moreover, three forces are coplanar. (under special circumstances, forces intersect at infinite and become a parallel force system). 3rd principle: the law of parallelogram Corollary 2: three equivalent forces intersect at one point R=F1 +F2 Two forces applied at one point of a body have as their resultant a force applied at the same point and represented by the diagonal of a parallelogram constructed with two given forces as its sides
学 公理3力的平行四边形法则 作用于物体上同一点的两个力可合成 R 一个合力,此合力也作用于该点,合力的 F2 大小和方向由以原两力矢为邻边所构成的 平行四边形的对角线来表示。 r=E+F R 推论2:三力平衡汇交定理 刚体受三力作用而平衡,若其中两力作 用线汇交于一点,则另一力的作用线必汇交 Fi F3 于同一点,且三力的作用线共面。(必共面, 在特殊情况下,力在无穷远处汇交—平行 力系。) 20
20 刚体受三力作用而平衡，若其中两力作 用线汇交于一点，则另一力的作用线必汇交 于同一点，且三力的作用线共面。（必共面， 在特殊情况下，力在无穷远处汇交——平行 力系。） 公理3 力的平行四边形法则 作用于物体上同一点的两个力可合成 一个合力，此合力也作用于该点，合力的 大小和方向由以原两力矢为邻边所构成的 平行四边形的对角线来表示。 推论2：三力平衡汇交定理 R=F1 +F2